1. Coat 8 x 4 x 2-inch loaf pan lightly with nonstick vegetable-oil cooking spray. Line with plastic wrap, smoothing out major wrinkles. Fill another loaf pan about three-quarters full with dried beans. Wedge loaf pan into beans so pan is tilted lengthwise.Bittersweet Layer:
2. Melt butter in small saucepan. Add chocolate, stirring until smooth. Stir in liqueur; cool. Fold in whipped cream. Pour into loaf pan, smoothing top with icing spatula; the top of mousse should form a diagonal across the pan. Place pans in freezer until mousse is firm, about 45 minutes.White Chocolate Layer:
3. Soften gelatin in water in small cup, 5 minutes. Heat together 1/4 cup cream and gelatin mixture in small saucepan to a bare simmer, stirring. Remove from heat. Stir in white chocolate until smooth. Let stand just until cool to touch, but still fluid.
4. Beat remaining 1/2 cup heavy cream in bowl until stiff peaks form. Fold into white chocolate mixture. Pour into tilted loaf pan; spread evenly. Freeze until firm, 30 minutes.Milk Chocolate Layer:
5. Melt chocolate in small saucepan over low heat, stirring until smooth. Remove from heat. Stir in liqueur. Let stand just until cool to the touch, but still fluid. Beat heavy cream in bowl until stiff peaks form. Fold cream into chocolate mixture.
6. Remove terrine from beans and place on flat surface. Pour in milk chocolate mixture; spread level. Cover and refrigerate until firm, about 2 hours. (Terrine can be frozen for up to 1 month.)To serve:
7. For easy slicing, freeze terrine for 1 hour. Unmold. Cut into thin slices. Transfer to dessert plates. If desired, garnish with shaved white chocolate and a dusting of unsweetened cocoa powder forced through a fine-mesh sieve.