1Heat oven to 350°F (325°F for dark or nonstick pan). Make cake as directed on box for 13x9-inch pan. Cool 10 minutes. Run knife around sides of pan to loosen cake; remove from pan to cooling rack. Cool completely, about 1 hour. For easier handling, refrigerate or freeze cake 30 to 60 minutes or until firm.
2Use serrated knife to cut cake as shown in diagram (for piano bench, cut 4 1/2x1 1/2-inch piece; cut horizontally in half to make bench lower than piano, if desired). Place cake on tray. Place about 2 tablespoons frosting in small resealable food-storage plastic bag. Frost cake with thin layer of frosting to set crumbs. Refrigerate or freeze 30 to 60 minutes to set frosting. Frost entire cake.
3Cut 1 bar white chocolate into 1 3/4x1/2-inch strips. From remaining bar, cut 2 1/2x1 1/2-inch piece for music. Cut milk chocolate into 1x3/8-inch pieces. Place white and milk chocolate pieces on cake for piano keys. Cut off tiny corner of bag with frosting; pipe frosting notes on white chocolate "music." Place above keys. Store loosely covered.