Put butter and brown sugar in oven and stir until well mixed; add pineapple slices.
In separate container, mix cake mix according to directions on package.
Pour the batter over pineapple and place lid on top.
Put in fire with coals on top and bottom and bake for 15 minutes.
Remove and continue to bake with coals on top only for 15-20 minutes longer.
Cool for 10 minutes before serving.