Break the graham crackers into individual rectangles (each graham cracker sheet should give you 4 rectangles).
Cut each rectangle in half crosswise to get 8 squares.
Cut marshmallows in quarters.
Set an oven rack at the top position (about 3 inches from the broiler element) and heat the broiler to high.
Place the graham crackers on an aluminum foil-lined rimmed baking sheet.
Place a chocolate square on each graham square, and cover the chocolate with a marshmallow square.
Place the s'mores in the oven and broil until the marshmallow is lightly browned, 15 to 30 seconds (watch your broiler closely as heat intensity varies).