Line 13x9-inch pan with Reynolds Wrap Aluminum Foil, with ends of foil extending over sides.
Microwave butter in large microwaveable bowl on HIGH 45 sec. or until melted.
Add sugar, graham crumbs and peanut butter; mix well.
Spread onto bottom of prepared pan; cover with chocolate.
Make shallow cuts in top of dessert to mark 48 squares. Refrigerate 1 hour or until firm. Use foil handles to remove dessert from pan; cut into squares following scored lines on dessert.