In a large saucepan, melt the butter over low heat. Add the marshmallows and stir until completely melted. Remove from the heat.
Gently stir in the popcorn until it’s fully combined. Stir in the Lucky Charms marshmallow bits, being careful not to over stir them (or the colors will bleed).
Once the mixture is cool enough to handle, grease your hands with cooking spray. Roll 24 balls and place them on a parchment-lined cookie sheet. Let them cool completely and then store them in an airtight container.
These popcorn balls are best if eaten the same day they're made.