Chocolate Truffle Layer Cake

 Let cool completely, about 30 minutes.

In the bowl of a standing electric mixer fitted with a wire whisk, beat the butter at medium speed until light and fluffy. Add the cooled chocolate mixture. At low speed, beat in the confectioners' sugar.

Cut out a 5-by-11-inch cardboard rectangle. Place a sheet of parchment or wax paper over each cake and top with a flat cookie sheet or cutting board. Invert the cakes and remove the pans. Keeping the paper attached, trim each cake to an even 15-by-11-inch rectangle, then cut each cake into three 5-by-11-inch rectangles; you will have 6 rectangles.

Spoon a small dollop of the chocolate frosting onto the cardboard rectangle and transfer one cake rectangle to it; peel off the paper. This

 will be the base. Spread half of the white chocolate ganache on the base and top with another cake rectangle; peel off the paper. Spread half of the dark chocolate ganache on the cake and top with another cake layer, peeling off the parchment. Spread with 1 1/4 cups of the chocolate frosting. Repeat with 2 more layers, peeling the parchment off the cake rectangles and spreading them with the remaining white chocolate ganache and dark chocolate ganache. Keep the sides even as you build the cake by smoothing them out with a metal cake spatula. Top with the final cake rectangle and peel off the parchment. Coat the sides and top of the cake with a smooth layer of chocolate frosting; refrigerate briefly to set the frosting.

Spread the remaining frosting over the cake and garnish with the chocolate shavings. Carefully transfer the cake (on its cardboard base) to a platter and refrigerate until firm. Using a hot knife, cut the cake into slices while cold and let come to room temperature before serving.