Spread popped popcorn onto a cookie sheet lined with wax paper. Remove any unpopped kernels.
Place butter, marshmallows and brown sugar in a large microwavable bowl. Microwave for 1 minute. Stir to combine, and then microwave another 30-60 seconds, until it’s all melted.
Pour hot marshmallow mixture over the popcorn and mix. (It is very sticky! Try spraying a spatula with non-stick spray to use when you mix it)
Let stand about 20 minutes for marshmallow mixture to set slightly.
Serve and enjoy! ♥