Build a campfire and allow it to burn down until you have hot, glowing coals. Avoid cooking over large flames for better control.
Place a cast iron skillet near the fire to warm slightly. Add butter and spread it evenly across the base and sides to prevent sticking.
Add the chocolate chips to the skillet and spread them into an even layer so they melt smoothly.
Scatter the mini marshmallows over the chocolate, covering most of the surface. Slight gaps are fine as they will expand during cooking.
Place the skillet over the campfire on a grill grate or stable surface above the coals. Cook for 5 to 8 minutes, rotating occasionally for even heat.
Watch closely as the chocolate melts and the marshmallows soften and turn golden brown. Move the skillet away from direct flames if needed.
Carefully remove the skillet from the fire using heat-safe gloves. Let it rest for 1 to 2 minutes.
Serve immediately with graham crackers for dipping while the mixture is warm and gooey.