Triple-Chocolate Layer Cake
1/2 C. cold buttermilk
1 T. instant espresso powder
2 t. vanilla extract
1 1/4 C. sugar
1 C. cake flour
6 T. unsweetened cocoa powder (preferably Dutch-process)
1/2 t. salt
1/4 t. baking powder
1/4 t. baking soda
3 large eggs
1/2 C. unsalted butter, room temperature
For fillings and frosting:
4 oz. good-quality white chocolate (such as Bakers of Lindt), finely chopped
4 oz. imported milk chocolate, finely chopped
1 C. plus 1 T. chilled whipping cream
6 oz. bittersweet (not unsweetened) or semisweet chocolate, finely chopped
White and milk chocolate shavings (optional)
To make cake: Preheat oven to 350°F. Butter three 8 inch diameter cake pans with 1 1/2 inch high sides. Line bottom of pan with parchment paper. Stir buttermilk, espresso powder and vanilla in small bowl until powder dissolves.
Sift sugar and next 5 ingredients into large bowl. Add eggs and butter; beat until mixture is thick and smooth, about 1 1/2 minutes. Add buttermilk mixture; beat until light and fluffy, about 1 1/2 minutes longer. Divide batter among pans.
Bake cakes until tester inserted into center comes out clean, about 20 minutes. Cool cakes in pans on racks 5 minutes. Turn cake out onto racks. Peel off parchment; cool completely.
To make fillings and frosting: Place white chocolate and milk chocolate in separate medium bowls. Bring 1 cup cream to boil in heavy medium saucepan. Pour 3 tablespoons hot cream into white chocolate and 3 tablespoons hot cream into milk chocolate. Add bittersweet chocolate to remaining cream in saucepan; stir until smooth. Stir white chocolate and milk chocolate fillings until smooth; chill until cold but not firm, about 15 minutes. Set aside bittersweet chocolate frosting.
Place 1 cake layer on platter. Using hand-held mixer, beat white chocolate filling until slightly stiff. Spread over cake layer. Top with second cake layer. Add remaining 1 tablespoon cream to milk chocolate filling and beat until smooth. Spread over second cake layer. Top with third cake layer. Chill cake until bittersweet chocolate frosting is firm enough to spread, about 30 minutes.
Stir frosting until smooth. Spread very thin layer over top and sides of cake to smooth surface and anchor chocolate shavings. Chill 30 minutes. Spread remaining frosting decoratively over cake. (Can be made 1 day ahead. Cover with cake dome and chill.)
Garnish cake with chocolate shavings, if desired, and serve.