Square Sponge Slab Cake

The kind of cake I want to eat all summer long

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Today I’d like to talk to you about cake, specifically this square sponge slab cake that I’ve been riffing on all year long. I’m not sure what I love more: the soft, creamy texture vaguely reminiscent of the not-too-sweet Asian bakery cakes I grew up eating; or the simple assembly method that vastly reduces the hands-on time it takes to make a layer cake.

The cake

My favorite cake of all time is my mom’s sponge cake (you can find the full recipe in my cookbook), a simple but gloriously fluffy affair. The original version is baked in a tube/angel food pan and is best enjoyed plain in fat wedges. But when baked as a thin sheet, its soft crumb and mellow sweetness make it the perfect canvas for creamy layer cakes. (I reduce the sugar slightly to compensate for the added sweetness from the fillings.)

For the filling I’ve gone with one of my tried-and-true back pocket recipes, an egg white pastry cream adapted from Bravetart. I particularly like using this cream when doing an infusion, as the whites provide a perfectly neutral backdrop (yolks are so flavorful on their own that they can muddy delicate flavors). Here I’ve opted for lightly toasted fig leaves, which impart notes of almond and coconut. (We have a fig tree in our yard so the leaves are easy for me to source, but if you can’t find them, sub a vanilla bean).

In addition to the cream, I like adding both a cake soak and thin layer of jam for additional moisture and brightness. The soak can be pretty much any flavorful liquid that complements your other flavors. (I’ve used strongly brewed teas, coffee, infused syrups, the liquid from canned fruit, etc.) Use whatever jam you have on hand; if it’s particularly sweet or stiff, mix in a couple squeezes of lemon juice.

Finally, the frosting: of course you can opt for a plain whipped cream (I often do a 1:2 ratio of sour cream:heavy cream plus sugar to taste, for a quick stabilized whipped cream). But I LOVE this light whipped white chocolate ganache — it’s just sweet enough, incredibly velvety, very stable, and wonderful to pipe.

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The method

To assemble this cake, I’ve borrowed from the commercial bakery practice of cutting out cake layers from a single flat sheet cake rather than baking each layer in a separate pan (If you’ve ever made a Milk Bar cake, you’ll be familiar with this process). You then layer your cake layers and fillings in your pan/mold, and let it all set for a few hours or overnight. Finally, you unmold the cake and finish as desired — no need to crumb coat as the rest time melds everything together nicely.

I love the combo of half sheet pan and 8x8 square pan because…efficiency! By using a half sheet pan, I can simply line it with a reusable silicone mat. This means spending literally zero minutes cutting parchment paper and prepping cake pans (one of my least favorite kitchen tasks). And one half-sheet pan of cake yields three neat layers that fit in my 8x8 (this one specifically) with very little waste. 

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One final thing: I hope you’ll use this recipe as a springboard for your own slab cake creations. I rarely make the same cake twice, using whatever I have around to inform the flavor of the moment. Play around with different infusions and soaks. Try adding a curd or a seasonal fruit compote instead of jam. Adapt your own favorite cake recipes to fit a half sheet. Here are a few different cakes I’ve made using this method:

Clockwise from top left: sponge cake / rooibos soak / rooibos cream / red currant jam / whipped cream; sponge cake / lime soak / mango namelaka / gooseberry-mango jam / creme fraiche whipped cream; genoise sponge / lychee soak / strawberry-lychee cremeux / lychee whipped ganache; carrot cake / caramelized white chocolate creme legere / extra strength brown butter cream cheese frosting

I’d love to see what you create: tag me on instagram (@rushyama) or leave a comment here!

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Square Sponge Slab Cake

Makes one 8x8x2 square cake

Ingredients:

For the sponge cake:

• 5 large eggs, separated

• 120g caster or granulated sugar, divided

• 55g neutral oil

• 1/4 tsp kosher salt

• 1 tsp vanilla extract

• 1/4 tsp cream of tartar

• 94g cake flour, sifted

For the fig leaf pastry cream:

• 5 medium fresh fig leaves

• 340g whole milk

• 225g heavy cream (35% milk fat)

• 130g granulated sugar

• 1/2 tsp kosher salt

• 34g cornstarch

• 120g egg whites (from about 4 large eggs)

• 28g unsalted butter

• 1 tsp vanilla extract

• 1/2 tsp almond extract

For the whipped white chocolate ganache:

• 2g powdered gelatin

• 10g cold water

• 270g heavy cream (35%), divided and cold

• 84g good quality white chocolate (28-30% cocoa solids), finely chopped

• 1 1/2 tsp vanilla extract

To finish:

• 105g elderflower soak (I either reconstitute the elderflower drink concentrate from IKEA and add half the amount of water suggested, or heat together 30g sugar and 60g water until the sugar is dissolved, then stir in 15g St-Germain liqueur)

• 120g seedless berry preserves/jam

• Fresh berries, edible / non-toxic flowers, sprinkles, for garnish

Method:

For the sponge cake:

Preheat the oven to 350F with a rack in the middle. Line the bottom of a half sheet (13x18) pan with a silicone mat or parchment paper. Do not grease the sides of the pan.

In a medium bowl, whisk together the egg yolks and half (60g) the sugar until paler in color and slightly thickened, about 2 minutes. Whisk in the oil, salt, and vanilla until smooth. Set aside.

In the bowl of a stand mixer fitted with the whisk attachment, beat the egg whites and cream of tartar on medium-low until foamy. Increase the speed to medium. When the whites reach soft peak stage, slowly add the remaining 60 grams (¾ cup) sugar, about 1 Tbsp at a time. Once all the sugar has been added, continue mixing on medium speed until the mixture is glossy and holds medium-stiff peaks.

Sift the flour into the yolk mixture in two batches, using a whisk or flexible spatula to mostly fold each portion in before sifting in the next. Once all the flour has been added, continue folding until all the flour is incorporated and the mixture is thick and smooth.

Add a large spoonful of the egg whites to the egg yolk mixture and fold it in using a whisk. Transfer the remaining egg white mixture to a large, wide bowl (the increased surface area makes folding the two batters together easier and faster). Scrape the egg-yolk mixture over the egg whites and use a whisk to fold the two batters together until mostly combined; then switch to a flexible spatula and continue folding until the batter is homogenous and no streaks of white remain. Be sure to scoop all the way down to the bottom of the bowl to make sure no pockets of unmixed batter remain.

Scrape the batter into the prepared sheet pan and level it with an offset spatula. (I like to pour the batter into the center of the pan, then push it out to the four corners, and finally spread it out to the remaining parts of the pan.)

Rap the pan firmly down on your countertop once or twice to pop any large air bubbles.

Bake for about 17-20 minutes, or until lightly browned, the center is springy to the touch, and when you press lightly on the cake it doesn’t sound like wet shoes on linoleum floor.

Rap the pan firmly on the countertop as before (this “shocks” the cake and prevents shrinkage), then transfer to a cooling rack. Run a thin bladed knife around the edges of the cake but allow it to cool in the pan completely. If not assembling the cake the same day, remove the cake from the pan, wrap well with plastic, and freeze until ready to use. (Cake can be frozen for up to 2 months.)

For the fig leaf pastry cream:

In a dry skillet over medium heat, toast the fig leaves for 1-2 minutes or until dry and fragrant, flipping occasionally. Set aside.

In a medium saucepan, combine the milk and cream. Heat over medium until steaming; then remove from heat, stir in the toasted fig leaves, and cover the pan. Let infuse for 1 hour.

Squeeze the fig leaves to release any dairy back into the saucepan, then discard the leaves. Bring dairy back to a simmer over medium heat. Meanwhile, in a medium bowl, whisk the salt and cornstarch into the sugar until well combined. Whisk in the egg whites.

Once the dairy reaches a simmer, remove from heat. Pour about half the dairy in a slow, steady stream into the egg white mixture, whisking constantly to avoid cooking the eggs. Scrape the warmed egg white mixture back into the saucepan. Cook over medium heat, whisking constantly, until the custard thickens and starts to bubble. Once bubbles appear, continue whisking the custard on the heat for one full minute to cook out any starchiness. Strain into the prepared container. Whisk in the butter and extracts. Press a piece of plastic wrap directly against the surface of the custard and cover. Refrigerate until cold, at least 2 hours (or up to 5 days).

For the whipped white chocolate ganache:

In a small heat-safe bowl, combine the gelatin and cold water. Let bloom for 5-10 minutes.

In a medium heatsafe bowl, combine the white chocolate and a third (90g) of the heavy cream. Microwave in 20 second intervals, stirring well in between bursts, until the chocolate is melted and the mixture is smooth.

Microwave the gelatin for 10 seconds to liquefy. Pour into the chocolate-cream mixture and stir well to combine. Add the remaining 180g cold heavy cream and stir well to combine. Transfer to a clean container, press a piece of plastic against the surface, and refrigerate at least 6 hours or up to three days.

When ready to use, whip on medium speed with a hand or stand mixer fitted with the whip attachment until the ganache reaches stiff peaks. Use immediately.

Assemble the cake:

Remove any skin/crust from the sponge cake. If the cake has been wrapped in plastic, usually most of the skin comes off with the plastic wrap. But if not, just gently rub it off with your hand.

Beat the pastry cream with a flexible spatula until smooth and spreadable.

Use the bottom of an 8x8 square cake pan to measure and cut out your cake layers. You should get two full squares along with two rectangles to form the third layer, like so:

Line your 8x8 pan with two pieces of criss-crossing plastic wrap, leaving plenty of overhang on all sides. Place a full square cake layer on the bottom of the pan (this will eventually be the top of your cake, so choose your nicest layer). Using a pastry brush, dab a third of the elderflower soak (about 35g) over the entire surface of the cake. Use an offset spatula to spread 60g jam evenly over the cake, followed by 350g pastry cream. Place the two rectangles of cake to form the second layer, then repeat the layering process (soak, jam, pastry cream). Brush the elderflower soak on the final layer of cake before placing it, soak side down, on top. (Take care as the added soak makes this layer more fragile to move!) Use the overhanging plastic wrap to cover the cake. Transfer to the refrigerator to chill overnight, or up to 2 days.

When ready to finish the cake, invert the cake onto a 10” cake board or serving plate (spread a little jam or cream in the center of the board to keep the cake from sliding). Use an offset spatula to spread an even layer of whipped ganache over the top and sides of the cake. Avoid overworking the ganache or it will turn grainy. Use any remaining ganache to decorate as desired — you can do some decorative piping, or simply swoop it casually over the top. Garnish with fresh fruit, edible or non-toxic flowers, sprinkles, etc. Keep refrigerated; temper at room temperature for 15-30 minutes before serving. Refrigerate leftovers in an airtight container for up to 5 days.