Line 13x9-inch pan with foil; spray with cooking spray. Place 15 grahams in pan, with sides overlapping slightly. Break remaining grahams into pieces.
Microwave butter and unsweetened chocolate in large microwaveable bowl on HIGH 1-1/2 to 2 min.; stir until chocolate is completely melted. 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.) Sprinkle with marshmallows and semi-sweet chocolate. Bake 3 to 5 min. or until marshmallows begin to puff. Press reserved graham pieces gently into marshmallows. Cool. Use foil to lift brownies from pan before cutting to serve.