Pop popcorn kernels in air popper or in microwaveable dish. DO NOT cook with oil. When popcorn is completely popped, separate popped kernels from unpopped and pour into buttered clean sink (DO NOT season). Melt stick of butter and bag of marshmallows on low heat (to prevent scorching). When done, pour over the top of the popped corn; stirring with buttered spatula or serving spoon, mixing well. Be careful! Mixture will be HOT! Taking handfuls at a time, form popcorn into popcorn balls. Wrap in plastic wrap or package in bags.