Beat pudding mix and milk in large bowl with whisk 2 min.
Add peanut butter; mix well. Stir in COOL WHIP.
Layer 1/3 of the grahams and half of the pudding mixture in 13x9-inch dish, breaking grahams as necessary to fit. Repeat layers. Top with remaining grahams.
Microwave chocolate and butter in microwaveable bowl on HIGH 2 min. or until butter is melted, stirring after 1 min. Stir until chocolate is completely melted and mixture is well blended.