Rake the coals of a campfire into a flat cooking surface.
Take each square foot sheet of aluminum foil, and rub with one tablespoon butter.
Cut the donuts in half (sandwich way), and lay one half on each sheet of aluminum foil. Place a pineapple slice onto each donut half, and spread with a tablespoon of brown sugar. Cover with the remaining donut halves and wrap tightly with the foil.
Cook the foil packets on the coals, turning occasionally until the donut has toasted and the pineapple is hot, about 10 minutes.
The foil packets can also be cooked on a grill heated to medium heat.