Heat oven to 350°F. Grease and flour 2 (9-inch) round cake pans. Line bottom of each pan with waxed or parchment paper, cut to fit.
Combine cake mix, milk, melted butter, egg whites, egg and vanilla in bowl; beat at medium speed until well mixed. Pour half of batter into bowl. Add melted white chocolate to 1 bowl and melted bittersweet chocolate to other bowl; stir each until well mixed.
Drop half of white batter by tablespoonfuls into each prepared pan; do not spread batter. Spoon half of chocolate batter into each pan, dropping it between spoonfuls of white batter. Using table knife, swirl batters to create marbled design. Smooth top of batter.
Bake 25-35 minutes or until toothpick inserted in center comes out clean. Let cool in pan 10 minutes. Invert cakes onto cooling rack. Remove waxed or parchment paper. Cool completely.
Beat 1 cup butter in large bowl at medium speed until creamy. Add powdered sugar, vanilla and enough half & half for desired frosting consistency. Beat at low speed until well mixed.
Place 1 cake layer, top-side down, onto large serving plate. Spread top with 1 cup frosting. Place second layer, top-side up, over frosted layer.
Trim 1/2-inch slice from 2 opposite sides of cake. Cut cake in half between trimmed sides (see Diagram 1).
Spread 1/2 cup frosting over cut-sides of cake. Turn cake pieces and place frosted cut-sides together, forming body of butterfly. Make 2 V-shaped cuts in each outer edge to create separation of wings (see Diagram 2). Freeze cake at least 1 hour. Frost cake with small amount of frosting to seal crumbs. Return to freezer 15-30 minutes or until frosting is firm.
Remove from freezer; frost cake. Decorate wings with candies or sugars as desired. Make 2-inch cut in end of licorice twist. Pull apart to create antenna. Place licorice in center of cake to form butterfly body. Pipe frosting along edges of cake, if desired.