The makeup of this cake is simple: a yellow sponge cake plus chocolate frosting. But of course, its structure is what makes it exciting! 12 (12!!!!) layers of sugary, chocolatey, celebratory goodness.
What was new in this cake for me was cooking super thin cake layers in sheet pans. I'm used to 2" cake pans, not paper-thin (pancake-thin?) cake layers. But the advantage is that your baking time is next to nothing -- only about 5 minutes per layer. With all of these thin layers, you're going to want to be super organized. Use a kitchen scale to weigh out your layers if you want, or just mentally map out which pan you'll use for each section before you start. I don't have a set of matching sheet pans, so I used what I had (one tiny one, one enormous one!), and made a 4x8.5" stencil that I cut the layers from once they were baked. This works just fine!
If you don't have any sheet pans, there is a way to bake the layers on parchment (I have not tried this!). Follow the link to the recipe on Smitten Kitchen below, and you'll find more info on how to do this.
Recipe from Smitten Kitchen
Makes one 12-layer 4x8.5" cake
7 large eggs, separated
3 large egg yolks
1 pound (3 1/2 cups or 455 grams) confectioners’ sugar, plus extra for dusting racks
3/4 cup (94 grams or 3 1/3 ounces) all-purpose flour
1 tablespoon lemon juice
1/8 teaspoon table salt
Frosting and filling
1/2 pound (8 ounces or 227 grams) semi- or bittersweet chocolate, coarsely chopped
1/2 pound (2 sticks or 226 grams) unsalted butter, cut into chunks
1 teaspoon vanilla extract
3 large egg yolks
2 tablespoons confectioners’ sugar
Prepare your cake pans: Assemble the cake pans you will need. Line the bottom of each with a sheet of fitted parchment paper, and butter and flour (or use a butter-flour spray) the parchment and sides of the pan. Tap out excess flour, if needed.
Make the cake: Preheat oven to 450°F and place a rack in the center of your oven. In the bowl of an electric mixer, beat 10 egg yolks for a few minutes at high speed, until pale and lemon-colored. Reduce speed and gradually add sugar, then increase the speed and beat the yolks and sugar until thick and glossy. Scrape bowl occasionally with rubber spatula. Reduce speed again and gradually add flour and salt; increase speed mix for 5 minutes more, then mix in lemon juice. Scrape bowl again with a rubber spatula. In a separate bowl with cleaned beaters, or by transferring your cake batter to a new bowl and washing it out and drying it with a long sigh, beat the 7 egg whites with a whisk attachment until they hold stiff peaks. Because your yolk mixture is more or less the thickness of spackle at this point, stir a few heaping spoonfuls of the whites into it to loosen the mixture, before folding in the rest of the whites in three additions. When you’re done, your batter will have transformed from a dry paste to a spreadable, foamy batter.
Bake your cake layers: Spread your batter in prepared pans; try to push the batter rather than pull it with an offset spatula, it will help keep the parchment from rolling up. If you have a digital scale and want to be super-fussy about making sure the layers are even, weighing the batter and dividing it out accordingly will do the trick. Spread the batter evenly to the edges with an offset spatula; be careful not to leave any holes.
Bake each layer for 5 minutes, or until golden with some dark brown spots. Thicker layers may take up to 2 additional minutes. When layer is baked, remove it from the oven and flip it out onto a cooling rack that has been dusted with a small amount of confectioners’ sugar. Carefully, gently remove parchment paper then flip cake back onto another lightly dusted cooling rack to finish cooling. It’s best to cool the layers right side up; the tops are the stickiest part.
Repeat with remaining layers. Dunk your cake batter bowl in water right away; that egg yolk-enriched batter dries quickly and was surprisingly hard to scrub off later! Layers will cool very quickly. Use 4x8.5" parchment stencil to cut out layers of cake. You should end up with 12.
Make the filling and frosting: Melt chocolate until smooth. Set aside to cool to room temperature, but of course not so cool that it hardens again. In the bowl of an electric mixer, beat butter until soft and smooth, scraping frequently. Add vanilla and 3 egg yolks. Add sugar and cooled chocolate, beating until thoroughly mixed and scraping as needed.
Assemble the cake: Place four strips of parchment or waxed paper around the outer edges of your cake plate. Place first cake layer on plate and spread chocolate on top and to edges with an offset spatula. The filling must be spread fairly thinly to have enough for all layers and the outsides of the cake. Repeat with remaining layers, stacking cake as evenly as possible.
Spread chocolate on outside of cake in a thin coat, just to cover and adhere the crumbs to the cake. Place cake in fridge for 30 minutes (or freezer for 5 minutes) to set the chocolate. Spread chocolate more thickly and smoothly to make a final exterior coat of frosting. Remove paper strips.
Chill cake until needed.
Do ahead: You can bake the cake layers ahead of time, freezing them between sheets of waxed paper, wrapped tightly in plastic. No need to defrost before assembling. The whole cake has kept for 4 days for us now, and keeps safely for a week.