PREHEAT oven to 350°F. Line 13x9-inch baking pan with foil, with ends of foil extending beyond sides of pan; grease foil. Place 15 of the graham squares in bottom of pan, overlapping slightly. Break remaining 5 graham squares into large pieces; set aside.
PLACE butter and chocolate in large microwaveable bowl. Microwave on HIGH 2 minutes or until butter is melted. Stir until chocolate is completely melted. Stir in sugar. Add eggs and vanilla; mix well. Stir in flour until well blended. Spread over graham squares in pan.
BAKE 30 to 32 minutes or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Sprinkle evenly with marshmallows and chocolate chunks. Bake an additional 3 to 5 minutes or until marshmallows begin to puff. Press reserved graham pieces gently into marshmallows. Cool in pan on wire rack. Lift out of pan onto cutting board using foil handles. Cut into 36 bars.