Place popped popcorn on a large baking sheet and remove any unpopped kernels or burnt pieces. Transfer clean popcorn to a very large bowl and set aside.

Melt butter over medium heat in a large saucepan. Keep stirring as the butter begins to foam and brown bits form on the bottom. Continue stirring until the butter turns from lemony yellow to deep golden color with a nutty aroma.

Turn heat to low and add marshmallows. Stir until the marshmallows have completely melted and mixture is smooth.

Pour the melted marshmallow mixture over the popcorn and stir using a rubber spatula until the popcorn is evenly coated.

While mixture is still warm, grease your hands with butter or oil and shape the popcorn mixture into balls. Press together gently but firmly so they hold their shape. Don't compact too tightly.

Once cooled, wrap each popcorn ball individually in plastic wrap and store in an airtight container.