Heat oven to 350 degrees F. Grease and flour three 9 x 2-inch round cake pans.

Microwave 4 ounces chocolate in glass bowl on high to melt, stirring once, 2 minutes. Stir in remaining chocolate. Cool slightly.

Sift flour, cocoa, baking soda, baking powder and salt onto waxed paper.

Beat butter and sugar in bowl until fluffy, 3 minutes. Beat in vanilla. Beat in eggs, one at a time. Beat in chocolate.

Mix sour cream and water in bowl. On low speed, beat flour mixture into butter mixture in 3 additions, alternating with sour cream, beginning and ending with flour; beat on medium-high speed 3 minutes. Spread 1/3 of batter in each pan.

Bake in 350 degrees F oven 25 to 30 minutes, until wooden pick inserted in center of comes out clean; do not overbake. Cool 15 minutes on rack. Turn cakes out onto rack. Cool.

Frosting: Chill 2-1/2 cups heavy cream, large mixing bowl a beaters in refrigerator. Bring additional 1/2 cup heavy cream in saucepan to boiling. Remove from heat. Stir 2 ounces chopped premium white baking chocolate until melted. Pour into second bowl; refrigerate to chill, about 1 hour. Add chilled cream, chilled chocolate mixture, 1 teaspoon vanilla and 6 tablespoons confectioners' sugar to chilled; beat 3 minutes or until stiff enough to spread, but not grainy. Sandwich 3 cake layers together, using 1 cup frosting between. Frost top sides with remaining frosting. Chill cake at least 2 hours for easy cutting. Garnish with white chocolate curls. Keep cake refrigerated until ready to serve.