Unwrap the peanut butter cups and place into a 10-inch cast iron skillet. Top with marshmallows.
Bake for about 8 minutes or until the marshmallows are golden brown. Carefully remove from the oven to a rack or pot holder. Serve immediately right from the pan (be careful, the pan is very hot) with graham crackers.
Cook's Note: This dip can be made in any 10 or 12-inch ovenproof skillet. Or you can use an 8-inch square baking dish or an 8-inch round cake pan in place of the cast-iron skillet.