1 HR 35 MIN
Prepare cake batter and bake in 13x9-inch pan as directed on package. Cool cake in pan 10 min. Pierce cake with large fork at 1/2-inch intervals.
Melt 3 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 all milk mixture is absorbed. Refrigerate 1 hour. Meanwhile, make curls from remaining chocolate. (See Tip.)
Frost cake with COOL WHIP. Garnish with chocolate curls. Keep refrigerated.