Line 9-inch square pan with foil, with ends of foil extending over sides; spray with cooking spray. Microwave butter in large microwaveable bowl on HIGH 30 sec. Stir in 2 cups marshmallows; microwave 45 sec. or until marshmallows are completely melted, stirring after 30 sec.
Add cereal, graham snacks and remaining marshmallows; mix well.
Press onto bottom of prepared pan; drizzle with chocolate.
Cool 1 hour. Use foil handles to lift dessert from pan before cutting into bars.
