PREHEAT grill to medium-low heat. Arrange 15 of the graham squares in single layer on bottom of 13x9-inch disposable foil baking pan; top each square with 4 of the marshmallow halves. Sprinkle evenly with the chocolate.
SPREAD each of the remaining 15 graham squares with 1 tsp. of the peanut butter. Place, peanut butter-sides down, on top of graham squares in pan to make 15 s'mores. Cover pan with foil; place on rack of grill.
COOK 4 to 6 min. or until marshmallows are puffed and chocolate begins to melt. Let stand 5 min. before serving.