Prepare cake batter and bake in 13x9-inch pan as directed on package. Cool cake in pan 10 minutes, then pierce cake with large fork at 1/2-inch intervals. Melt 7 ounces chocolate as directed on package. Blend milks and sour cream in blender until smooth. Add melted chocolate; blend well. Pour over cake, re-piercing cake if necessary until milk mixture is absorbed. Refrigerate 1 hour. Meanwhile, make curls from remaining chocolate. Stir cocoa into Cool Whip Frost cake with Cool Whip mixture; top with chocolate curls. Keep refrigerated.