Make this for our Chocolate Mousse Cake.
Preheat oven to 400 degrees. Butter a 12-by-17-inch rimmed baking sheet, line with parchment paper, and butter the paper. Combine 1 tablespoon flour and 1 tablespoon cocoa, and coat the pan, tapping out excess. Place the egg yolks in the bowl of an electric mixer fitted with the whisk attachment. Beat on high speed until pale yellow, about 5 minutes. Transfer to a medium bowl, and set aside. Wash and dry the mixer bowl.
Place the egg whites in the clean mixer bowl; with the whisk attachment, beat on medium speed until soft peaks form, about 3 minutes. Increase the speed to medium-high, and add the sugar gradually, until glossy stiff peaks form, about 3 minutes. Transfer the egg-white mixture to a large bowl.
Using a rubber spatula, fold the egg-yolk mixture into the egg-white mixture. Sift the flour and cocoa powder over the top, and gently fold them into the batter. Gently pour the batter into the prepared baking sheet, and smooth the top with an offset spatula.
Bake until cake springs back when touched, 10 to 12 minutes. Remove from oven; immediately turn cake out onto wire rack lined with parchment. Peel parchment from cake; cool completely. Use genoise the day it is made.