In a heavy small saucepan, melt the 6 ounces bittersweet chocolate and the butter. Cool slightly.
In a large mixing bowl, beat egg yolks and the 2 tablespoons sugar with electric mixer on medium to high speed for 2 to 3 minutes or until eggs are pale yellow. Gradually beat in the chocolate mixture until combined. Sprinkle flour over mixture, but don't stir.
In a medium bowl, beat the egg whites with an electric mixer on medium speed for about 1 minute or until soft peaks form (tips curl). Gradually add the second 2 tablespoons sugar, 1 tablespoon at a time, beating on high speed until stiff peaks form (tips stand straight). Gently fold into the chocolate mixture.
Pour batter into an ungreased 8-inch springform pan. Bake in a 350 degree F oven for 20 to 25 minutes or until cake springs back when you touch it lightly. Remove the cake from the oven and cool in the pan on a wire rack.
Loosen cooled cake from sides of pan. Remove sides of pan. Slice cake in half horizontally. Set the top half of cake aside. Reassemble the springform pan around bottom half of cake.
In a heavy small saucepan, combine milk chocolate and the first 1/2 cup whipping cream. Heat over medium-low heat. Stir until chocolate is melted and mixture is smooth. Remove from heat. Stir in orange liqueur, if you like. Cool slightly. Pour milk chocolate mixture over cake in springform pan and spread to cover top of cake. Place in freezer at least 45 minutes or until chocolate has set.
In a heavy small saucepan, combine white chocolate and the 1/3 cup whipping cream over medium-low heat. Stir until chocolate is melted and mixture is smooth. Remove from heat. Stir in hazelnut liqueur, if you like. Cool slightly. Pour white chocolate mixture over milk chocolate layer, spreading to cover. Return to freezer for at least 45 minutes or till set.
In a heavy small saucepan, combine the 12 ounces bittersweet chocolate, the second 1/2 cup whipping cream and corn syrup over medium-low heat. Stir until bittersweet chocolate is melted and mixture is smooth. Remove from heat and cool slightly. Pour over white chocolate layer; spread to cover layer.
Immediately top with the reserved cake layer. Cover the cake and return to the freezer for at least 45 minutes or until chocolate layer is set. Transfer to refrigerator. Chill for at least 2 hours.
To serve, loosen cake from sides of pan. Let stand for 30 minutes before serving. Sprinkle the top with cocoa powder and powdered sugar. Garnish with raspberries and mint, if you like. Slice cake. For easier cutting, dip knife in warm water between slices. Makes 20 servings.
