Preheat oven to 350 degrees F. Grease four 5-inch cast iron skillets with butter. Set aside.
In a large heatproof bowl combine butter, chocolate chips, cocoa powder, and coconut oil. Place over a pot filled with 2-inches simmering water, making sure the bowl does not touch the water. Heat mixture until butter and chocolate have melted. Stir as needed to ensure even heating. Once mixture is smooth, remove from heat. Allow melted mixture to cool for 10 minutes.
Stir in eggs, vanilla, and sugar. Once eggs have been incorporated, add flour, baking powder, and salt. Fold to combine. Add chocolate chips, nuts, and marshmallow. Fold until toppings have been evenly distributed.
Divide brownie batter evenly among four prepared cast iron skillets. Place skillets on baking sheet tray. Bake for 18-20 minutes until the tops of the brownies are set. Do not overbake. Brownies will continue to cook in skillet out of oven. Allow to cool slightly and top with ice cream of choice. Serve immediately.