PREHEAT oven to 350°F Place 9 of the graham squares on bottom of 9-inch square baking pan. Top with 36 of the marshmallow halves; sprinkle evenly with chopped chocolate. Cover with remaining 9 graham squares.
ARRANGE 4 marshmallow halves, cut sides down, on top of each graham square.
BAKE 9 to 11 minutes or until marshmallows are puffed and golden brown. Let stand 5 minutes. Cut into 18 bars with greased knife. Serve immediately.