Step 1Using a double-boiler (or in a bowl set over simmering water), melt the butter and chocolate, gently stirring, to form the ganache. Hold in a warm place until ready to cover the cake.
Step 1Heat the oven to 375 degrees. Grease a 13-by-9-inch baking pan.
Step 2Using a double-boiler (or in a bowl set over simmering water), combine the chocolate, cocoa and butter, gently stirring until melted. Remove from heat.
Step 3In the bowl of a stand mixer, or in a large bowl using an electric mixer, beat the egg yolks with the sugar until the yolks are very light in color and increased in volume, 6 to 10 minutes. Separately, beat the egg whites until stiff peaks form, 4 to 6 minutes.
Step 4Carefully fold the melted chocolate into the beaten egg whites until combined, then fold in the egg yolk mixture. Fold in the coffee liqueur.
Step 5Place the mixture in the prepared pan. Bake the cake until a toothpick inserted comes out clean, about 40 minutes. The cake will puff as it bakes and then will settle after it comes out of the oven and cools.
Step 6Cool the cake completely on a metal rack, then drizzle with ganache. Serve garnished with whipped cream or crème fraîche.