HEAT oven to 350°F.
PLACE 9 graham squares on bottom of 9-inch square pan. Top with 36 marshmallow halves; sprinkle with chocolate. Cover with remaining graham squares.
ARRANGE 4 marshmallow halves, cut sides down, on top of each graham square.
BAKE 9 to 11 min. or until marshmallows are puffed and golden brown. Let stand 5 min.