HEAT oven to 350°F.
LINE 13x9-inch pan with foil, with ends of foil extending over sides; spray with cooking spray. Cover bottom with 15 grahams, overlapping as necessary to form even layer. Break remaining grahams into small pieces.
MICROWAVE butter and unsweetened chocolate in large microwaveable bowl on HIGH 2 min. or until butter is melted; stir until chocolate is completely melted and mixture is well blended. Add sugar, eggs and vanilla; mix well. Stir in flour. Pour over grahams in pan.
BAKE 30 to 32 min. or until toothpick inserted in center comes out with fudgy crumbs. (Do not overbake.) Top with marshmallows and chocolate chunks. Bake 3 to 5 min. or until marshmallows begin to puff.
TOP with graham pieces; press gently into marshmallows with spatula to secure. Cool. Use foil handles to lift brownies from pan before cutting to serve.