A dark chocolate génoise (sponge cake) brushed with a little coffee and cognac makes the four layers. A milk-chocolate mousse hugs the layers together and a white-chocolate mousse gently envelops the whole chocolate cake. As a last tease, bittersweet chocolate curls dot the cake and add yet another layer of chocolate flavor.
*Note: To set up a double boiler, place a nonreactive stainless steel or glass bowl over a pot filled with water large enough so that the bottom of the bowl just barely touches the surface.
**Note: The cakes can be prepared up to 24 hours ahead up to this point. Cool to room temperature, cover in plastic wrap, and refrigerate. Bring to room temperature before assembling the cake.
Place the chocolate pieces in a double boiler.* Bring the water to a simmer over medium-low heat and melt the chocolate, stirring constantly until smooth. Remove from heat and let cool until just warm to the touch.
Place the heavy cream, chocolate liqueur, and vanilla extract in the bowl of an electric mixer. Whip at medium speed until thick, taking care not to overbeat or it will turn to butter. Pour the melted milk chocolate into the whipped cream and fold until well blended. Transfer to a container, cover, and refrigerate for 2-4 hours.
Place the white chocolate pieces in a double boiler. Bring the water to a simmer over medium-low heat and melt the chocolate, stirring constantly until smooth. Remove from heat and let cool until just warm to the touch.
Place the heavy cream in the bowl of an electric mixer. Whip at medium speed until thick, taking care not to overbeat or it will turn to butter. Pour the melted white chocolate into the whipped cream and fold until well blended. Transfer to a container, cover and refrigerate for 2-4 hours.
Preheat the oven to 375 degrees.
In a bowl, place the flour, cocoa, baking powder, and salt. Whisk until well blended. Place the eggs in the bowl of an electric mixer. Whisk at high speed until very pale, thick, and ribbony, about 2 minutes. Slowly add the sugar and continue whisking at high speed until soft peaks form, about 1 minute.
Add ½ of the flour-cocoa mixture to the whipped eggs. Fold in carefully and add the melted butter and fold in carefully. Fold in the remaining flour-cocoa mixture, taking care not to overmix.
Lightly grease two 9-inch-round nonstick cake molds and dust with flour. Divide the batter equally into the molds. Bake for 18-20 minutes until the cakes have risen and a skewer inserted in the center comes out clean. Remove from the oven and let cool to room temperature before unmolding.**
Unmold the chocolate cakes and cut into four 1-inch-thick layers (it will be easier with a cake leveler). Place 1 layer of cake in the center of a serving platter. Line the rim of the platter with plastic wrap (this will keep the rim clean as you apply the frosting).
Using a pastry brush, soak the cake with ¼ of the coffee and cognac. Top with a 1/3 of the milk-chocolate mousse and spread it evenly over the whole surface of the cake. Top with another layer of the chocolate cake. Repeat soaking with the coffee and cognac, and spreading the milk-chocolate mousse 2 more times until you have 3 layers. Top with the last layer of chocolate cake and soak with the remaining coffee and cognac.
Apply the white chocolate frosting, first on the top, then on the sides. Save a little frosting for piping, if desired: Take a pastry bag and insert a pastry tip into the tip of the bag. Cut an opening large enough for the tip to fit through. Take the top of the bag and fold it over to form a rim. Holding the bag in one hand like an ice cream cone, transfer the remaining frosting to the bag using a rubber spatula and unfold the rim. Twist until the frosting is pushed toward the tip of the bag.
Hold the bag firmly at the top with one hand and the end of the bag in the other and pipe a rim around the top edge, squeezing from the top. Remove the plastic wrap lining the platter and pipe the remaining frosting around the base of the cake.
Loosely cover the cake with plastic wrap and refrigerate for 4-6 hours. Remove from the refrigerator 30 minutes before serving. When ready to serve, shave the bittersweet chocolate using a vegetable peeler over the cake and around the rim of the platter.