1. Preheat the grill on high.
2. Heat a cast-iron skillet over medium heat. Melt the butter in the skillet, add the nuts and toast until fragrant. Remove from the heat and sprinkle the chocolate chips evenly over the nuts. Spread the marshmallows over the chocolate chips.
3. Place the skillet under the grill until browned on top.
4. Serve in the skillet and use digestive biscuits for dipping.