Place a 9-inch cast iron skillet in the oven and preheat the oven to 450 degrees.
When the oven has preheated, carefully remove the hot pan from the oven, add the butter, and swirl the pan to coat with the butter. Add the chocolate chips in an even layer and place the marshmallow in concentric circles over the chocolate. Place back in the oven for 7-9 minutes or until the marshmallows are puffed and golden brown.