Preheat the oven to 300 degrees F. Line a rimmed jelly roll pan with parchment paper. Lightly spray with nonstick baking spray.

Place the popcorn on the prepared pan and set aside.

In a large microwave safe bowl, add the brown sugar, butter, peanut butter, and marshmallows. Place in the microwave for 1 minute and then stir until the mixture is completely melted. If it’s not completely smooth, microwave in 15 second increments and stir after each increment until no lumps remain.

Working quickly, pour the marshmallow mixture over the popcorn. Stir until the popcorn is well coated.

Separate the popcorn into a single layer if it’s clumpy.

Place in the preheated oven and bake for 30 minutes, stirring every 10 minutes. Continue to break apart the clumps of popcorn when stirring.

Allow to cool completely before serving.

Store in an airtight container for up to 1 week.