Heat oven to 350°F. Grease and flour 13x9-inch pan. Prepare and bake cake mix as directed on package, using water, oil and eggs. Cool 15 minutes. Carefully invert cake onto wire rack. Cool 45 minutes or until completely cooled. Wrap; freeze 2 hours or until firm.
Cover 17-inch square of heavy cardboard with foil. Reserve 2/3 cup chocolate frosting in small bowl; reserve 1 tablespoon chocolate frosting in resealable small plastic bag. In large bowl, combine remaining chocolate frosting and the vanilla frosting; blend well.
Cut and assemble frozen cake pieces on foil-covered cardboard. Frost top and sides of head with chocolate-vanilla frosting mixture. Frost antlers with reserved 2/3 cup chocolate frosting.
Decorate with gumdrops and candy-coated chocolate pieces for nose and eyes. Cut small corner off bag of reserved chocolate frosting; pipe eye lashes on reindeer.