LINE 8-inch square pan with foil, with ends extending over sides of pan. Beat margarine and peanut butter with mixer until well blended. Add wafer crumbs; mix well. Gradually beat in sugar; press onto bottom of prepared pan.
MICROWAVE Cool Whip and chocolate in microwaveable bowl on HIGH 1 min.; stir. Microwave 15 to 30 sec. or until chocolate is melted and mixture is blended; stir. Spread over peanut butter layer.
REFRIGERATE 2 hours. Use foil handles to lift dessert from pan before cutting into bars.