PREPARE cake batter and bake as directed on package for 13x9-inch pan, lining pan with foil sprayed with cooking spray before adding batter. Cool cake in pan 15 min.; invert onto wire rack. Remove pan; cool cake completely. Cut cake into pieces and arrange on platter as shown in diagram for the piano and piano bench; set aside. Discard cake trimmings or reserve for snacking.
BEAT dry pudding mix, sugar and milk with whisk 2 min. Stir in 1 cup COOL WHIP; use to frost piano and bench pieces. Spread remaining COOL WHIP into rectangle on top of piano as shown in photo.
MELT chocolate as directed on package; pour into resealable plastic bag. Seal bag. Cut small piece off 1 bottom corner of bag; use to draw piano keys on cake. Use any remaining chocolate to draw small music notes on biscuit; place on cake for sheet music. Refrigerate 30 min. or until glaze is firm. Keep cake refrigerated.