From Tasty.

Step aside, crispy rice treats! Holiday Marshmallow Popcorn is the new festive snack star. 1. Melt 1-12 oz bag of marshmallows and 4 tbsp butter over low heat until smooth. 2. Fold in 4 cups of popped popcorn, stirring to coat evenly. 3. Press the sticky mixture into a greased bowl or mold, sprinkle with colorful sprinkles, let it set, and enjoy your marshmallow popcorn treat!