In the middle of a 18" x 18" piece of aluminum foil, place 1 teaspoon of oil and 1 tablespoon of popcorn. Bring foil corners together to make a pouch. Secure the edges of the foil but leave plenty of room for the popcorn to pop. Tie the pouch to a stick and hold the pouch over the hot coals. Shake constantly until all the popcorn has popped. Add butter and salt to taste.