Grease a large baking sheet. Melt the butter in a large, heavy pot over medium heat. Add the marshmallows. Salt and cook until completely melted and smooth, stirring often.
Add the popcorn to the melted marshmallow mixture and stir until well coated. Scrape mixture onto the prepared baking sheet and let it cool slightly.
Once the mixture is cool enough to handle but still warm, used buttered hands to form the popcorn into 8 equal-sized balls. Let cool completely before eating.
