Set up box oven. See this post (http://angelaskitchen.com/2013/07/02/the-great-gluten-free-campout-box-oven-muffins-and-eggs/) for instructions. While charcoal is flaming and heating up, make gluten free brownie mix according to instructions on box.
Oil foil pan. Pour batter into pan, patting out batter to cover the entire pan. Sprinkle crushed gluten free graham crackers evenly over batter, then add mini-marshmallows.
When coals are ready, place brownies on cans in box oven. Set the box over brownies and allow to bake for 5 minutes longer than the longest bake time on the brownie batter box (in the case of Better Batter Brownie Mix, allow to bake for 30 minutes) before checking to see if brownie bars are done. Bars are done when bakes through in center and marshmallows have melted and browned slightly.
Remove brownies from box oven and allow to cool before slicing.
To bake these at home, bake in a 350 degree F oven for the shortest time on your baking mix box before checking for doneness.