Satisfy any sweet tooth with this decadent dessert from award-winning pastry chef Alain Roby of the Chicago Hyatt. On"The Martha Stewart Show," Alain and Martha used a larger version of this recipe to bake a cake that was more than 6 feet tall.
Preheat oven to 350 degrees. Butter two 10-by-3-inch round cake pans; set aside.
In a medium bowl, mix together baking soda and boiling water; add melted unsweetened chocolate and stir to combine. Set aside to cool.
In the bowl of an electric mixer fitted with the paddle attachment, cream together butter and sugar. Add eggs, one at a time, mixing well after each addition. With the mixer on low, add flour and 6 tablespoons cocoa powder; mix until combined. Slowly add buttermilk; mix until combined. Add chocolate mixture and vanilla; mix until well combined.
Divide mixture evenly between prepared cake pans. Transfer to oven and bake until a cake tester inserted into the center of the cake comes out clean, 45 to 50 minutes. Transfer cake pans to racks and let cool completely in pans.
Meanwhile, heat heavy cream and semisweet chocolate in a medium saucepan over medium heat until chocolate has melted and mixture is well combined. Let frosting cool until creamy in texture.
Run a knife around the edges of cake pans and invert cakes onto wire racks. Slice each cooled cake horizontally into three even layers. Set one layer on a rack set over a rimmed baking sheet. Pour some of the frosting over the first layer, smoothing with a spatula. Add a second layer and repeat process. Repeat process with remaining layers and frosting; smooth frosting over top and sides. Place remaining 6 tablespoons cocoa powder in a fine-mesh sieve; dust cake evenly with cocoa powder. Let cool completely before serving. Garnish with raspberries, mint, and whipped cream, if desired.