PREHEAT oven to 350ºF and spray a 13 x 9-inch baking pan with cooking spray.
ARRANGE 15 graham cracker halves on the bottom of the pan. Top each square with 4 marshmallow halves and sprinkle with chocolate. Spread peanut butter on remaining squares and place them peanut butter side down on top of the marshmallows and chocolate.
BAKE for 9 to 11 minutes until marshmallows puff and chocolate melts. Store leftovers on a plate covered in foil or in an air tight container and enjoy a cool treat on a hot day.