PREPARE cake batter and bake in 13x9-inch pan as directed on package (replace the oil in the directions for applesauce) (use cooking spray on pan instead of shortening or butter). Cool cake in pan 10 min., then pierce cake with large fork at 1/2-inch intervals.
MELT 7 oz. 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.