From America's Test Kitchen Season 4: Easy Sheet Cakes
Sheet cakes, for all their simplicity, can still turn out dry, sticky, or flavorless and, on occasion, can even sink in the middle. We wanted to find a simple, dependable recipe, one that delivered a moist yet also tender and chocolaty cake.
We started with the mixing method—testing everything from creaming butter to beating yolks, whipping whites, and gently folding together everything in the end. The best of the lot was the most complicated to make, so we took a step back. The simplest technique we tried was simply whisking all the ingredients together without beating, creaming, or whipping. The recipe needed work, but the approach was clearly a good match for the simple all-purpose nature of a sheet cake. First we added buttermilk and baking soda to lighten the batter, since the original cake had been too dense. To increase the chocolate flavor, we reduced the sugar and flour, and decreased the butter. To further deepen the chocolate taste, we used semisweet chocolate in addition to the cocoa. We baked the cake at a low temperature for a long time—40 minutes—to produce a perfectly baked cake with a lovely flat top. Though this cake can be frosted with almost anything, we like a classic American milk chocolate frosting, which pairs well with the darker flavor of the cake.
Makes one 9 by 13-inch cake
Melting the chocolate and butter in the microwave is quick and neat, but it can also be done in a heatproof bowl set over a saucepan containing 2 inches of simmering water. We prefer Dutch-processed cocoa for the deeper chocolate flavor it gives the cake. The baked and cooled cake can simply be served with lightly sweetened whipped cream or topped with any frosting you like.