Begin by popping enough plain, unsalted popcorn to fill a large bowl. Let it cool while you prepare the caramel.

In a saucepan, combine butter, brown sugar, and corn syrup. Heat on medium, stirring until it boils, about 4-5 minutes. Stir for 1 minute after boiling.

Add marshmallows to the caramel mixture. Stir until melted and smooth, about 2-3 minutes.

Transfer cooled popcorn into a mixing bowl. Pour the caramel-marshmallow mixture over the popcorn and mix until coated.

Spread coated popcorn on a lined baking sheet and cool for 30-45 minutes until the caramel hardens slightly.

Once cooled, break the popcorn into clusters or pieces as desired. Store in an airtight container.