HEAT grill to medium-low heat.
ARRANGE 15 graham squares in single layer on bottom of 13x9-inch disposable foil baking pan; top each square with 4 marshmallow halves. Sprinkle with chocolate.
SPREAD remaining graham squares with 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.
GRILL 4 to 6 min. or until marshmallows are puffed and chocolate begins to melt. Cool 5 min. before serving.