In a medium saucepan, melt the marshmallows and butter together over med-low heat until well combined. In a large bowl, pour the popcorn and M&M's. Pour the melted marshmallows over and stir with a rubber spatula that has been sprayed with cooking spray. Spray your hands with cooking spray to coat, then gather up handfuls of the popcorn and squish it into balls. Let them sit until they have hardened up, at room temp. Store in an airtight container whatever is not eaten right away.