Pop the popcorn using your preferred method (air popper, stovetop, or microwave).
Making Caramel
In a saucepan over medium heat, melt the butter.
Stir in the brown sugar and corn syrup. Bring to a simmer for about 5 minutes, stirring constantly.
Add the marshmallows and stir until they melt completely into a smooth sauce.
Coating Popcorn
Pour the caramel-marshmallow sauce over the popcorn and gently toss to coat.
Baking
Spread the coated popcorn on a baking sheet lined with parchment paper.
Bake at 250°F (120°C) for 30 minutes, stirring every 10 minutes to ensure even coating.
Serving
Let the popcorn cool completely before breaking it into pieces and serving.
Notes
Store in an airtight container at room temperature to maintain crunchiness. Avoid refrigeration to prevent chewiness. For added flavor, a pinch of salt can enhance the sweetness.