Use this chocolate genoise recipe to make our Buche de Noel.
Heat oven to 350 degrees. Butter a 10 1/2-by-15 1/2-by-1-inch jelly-roll pan. Line with parchment; butter and flour paper, tapping out the excess flour.
Sift flour, cocoa, and baking soda together twice into a medium bowl. Set aside. In a small saucepan over low heat, melt butter. Skim off white foam, and pour clear yellow butter into a bowl, discarding white liquid at the bottom. Set aside in a warm place.
In a medium-size heat-proof bowl, whisk together eggs and sugar. Set bowl over a pan of simmering water; stir until mixture is warm to the touch and sugar has dissolved. Remove from heat, and beat on high speed until mixture is thick and pale and has tripled in bulk. Reduce speed to medium, add vanilla, and beat 2 to 3 minutes more.
In three additions, sift flour mixture over egg mixture, folding in gently with a spatula. While folding in last addition, dribble melted butter over batter and fold in.
Spread batter evenly in pan, leaving behind any unincorporated butter in the bottom of the bowl. Tap pan on counter to remove air bubbles. Bake until cake springs back when touched in center, 15 to 20 minutes. Don't overbake or cake will crack. Let sit in pan on a wire rack until cool enough to handle.
Dust surface with cocoa powder. To make rolling easier, trim edges of cake, and cover with a sheet of waxed paper and a damp dish towel. Invert onto a work surface, and peel off parchment; dust with cocoa. Starting from one long end, carefully roll up cake in towel. Wrap in plastic; refrigerate until ready to use.