Adjust the oven rack to the center position of the oven. You want an even heat all around the skillet. Place the cast iron skillet on the rack, and preheat the oven to 450ºF. Once the oven is hot, use a pot holder or towel to remove the hot skillet from the oven.
Place oil in the skillet and, using a pot holder to hold the handle, swirl the oil in a layer on the bottom of the skillet to coat evenly. Place skillet on a pot holder and pour chocolate chips in an even layer. Carefully arrange peanut butter cups in one even layer over the chocolate chips. Arrange marshmallow halves over the peanut butter cups, uncut side up, until the skillet is covered completely.
Bake for 5-7 minutes or until the marshmallows are toasted to your preference. Watch closely. Use a pot holder or oven mitt to remove the skillet from the oven and allow to rest for 5 minutes before serving.
Serve immediately with graham cracker squares or sticks, but be careful because the dip and the skillet will be hot.