Adjust rack to center position of oven and place 8-inch cast iron skillet on rack. Preheat oven to 450°F with skillet inside. Once oven is preheated, use a pot holder to remove hot skillet from the oven. Place pat of butter in the skillet and use a pot holder to hold the handle and swirl the skillet so that the melted butter coats the bottom and sides. Pour chocolate chips in an even layer into the bottom of the skillet. Taking care not to burn fingers on the sides of the skillet, arrange marshmallow halves over the surface of the chocolate chips, covering the chocolate completely. Bake for 5 to 7 minutes or until marshmallows are toasted to your preference (watch closely!). Use an oven mitt to remove the skillet from the oven and allow to rest for 5 minutes. Serve immediately with graham cracker squares or sticks...but be careful, because the dip (and the skillet) will be very hot!