PREPARE cake batter and bake in 13x9-inch pan as directed on package; cool completely.
BEAT cream cheese and sugar in large bowl with mixer on medium-low speed until well blended. Add peanut butter; mix well. Blend in 1 cup COOL WHIP; spread onto cake.
PLACE chocolate in medium microwaveable bowl. Microwave on HIGH 2 min. or until completely melted, stirring with whisk after each minute. Stir in remaining 2 cups COOL WHIP; spread over cake. Refrigerate until set. Store in refrigerator.