How to Make It
Preheat the oven to 350° and position 2 racks in the middle and lower thirds. Butter three 9-inch round cake pans and line the bottoms with parchment or wax paper. Butter and flour the parchment.
In a medium bowl, whisk the cocoa with the water until smooth; let cool to room temperature. In a small bowl, combine the egg yolks, eggs and vanilla with one-fourth of the cocoa mixture.
In a standing electric mixer, stir together the flour, sugar, baking powder and salt. Add the butter and remaining cocoa mixture and mix at low speed just until the dry ingredients are moistened. Scrape down the bowl and beat at medium speed until light and smooth. Add the egg mixture in 3 batches, beating for 20 seconds between additions.
Divide the batter evenly among the cake pans. Smooth the tops and bake for about 25 minutes, or until a toothpick inserted in the center of each cake comes out clean. Let the cakes cool in the pans for 10 minutes, then turn each out onto a rack and peel off the paper. Invert onto another rack and let cool completely.
Center a cake layer on a platter and spread 3/4 cup of the Espresso Ganache over the top. Set a second cake layer on top of the first and spread 3/4 cup more filling on top. Cover with the third cake layer and spread the top and side with the remaining ganache.