Line 2 (9-inch) square cakepans with aluminum foil; grease foil.
Cook semisweet chocolate and butter in a small saucepan over low heat, stirring until blended. Cool.
Beat 2 cups sugar and 8 eggs at medium speed with an electric mixer 3 minutes or until foamy. Gradually add chocolate mixture, beating at low speed until blended. Pour into 1 prepared pan.
Bake at 325° for 1 hour or until set. Cool on a wire rack. Cover and chill 8 hours. Remove from pan, discarding foil. Trim edges, if necessary.
Beat cream cheese at medium speed with an electric mixer until creamy. Add remaining 1 cup sugar and flour, beating well. Add remaining 3 eggs, 1 at a time, beating just until blended after each addition. Stir in sour cream, white chocolate, and vanilla. Pour into remaining prepared pan.
Bake at 325° for 1 hour. Turn oven off. Leave cake layer in oven, with oven door partially opened, 30 minutes. Remove cake from oven; cool in pan on a wire rack. Cover cake, and chill 8 hours. Remove from pan, discarding foil.
Reserve 1 cup White Chocolate Ganache.
Place chocolate layer, bottom side up, on a serving plate. Spread with 1/2 cup White Chocolate Ganache, and top with white chocolate layer, bottom side up. Spread top and sides of cake with remaining ganache.
Stir desired amount of food paste into reserved 1 cup ganache.
Insert metal tip No. 2 into a large decorating bag; fill with ganache. Pipe lace design (continuous string of frosting without touching) on top and sides of cake.
Arrange 2 (17- x 2 1/2-inch) White Chocolate Ribbons on cake. Form bow with 1 (13- x 2 1/2-inch) strip; arrange 2 (4- x 2 1/2-inch) strips and 2 (3- x 2 1/2-inch) strips for bow ends. Place 1 (2 1/2- x 1-inch) strip in center of bow for knot.
Store cake in refrigerator. Remove from refrigerator, and let stand at room temperature 1 hour before serving. Garnish, if desired.