Line a 9x13 pan with foil, leaving an overhang on the sides, and lightly grease with cooking spray; set aside.

Pop the popcorn and mix in the peanuts; set aside.

Melt butter over medium-high heat.

Add peanut butter and marshmallows; stir until melted and smooth.

Pour over popcorn and mix until thoroughly combined.

Transfer popcorn mixture to the prepared pan.

Using a greased spatula, or wet hands, press mixture into the pan.

Let cool completely.

Lift out the popcorn and cut into bars.

Serve immediately.