Measure 8 cups of salted, popped popcorn into a large bowl. Spray a baking sheet with cooking spray.
Add the melted butter to a small pot then add the marshmallows. Mix the butter and marshmallows together over low (to medium-low) heat until the marshmallows are completely melted and the mixture is fully combined.
Pour the marshmallow mixture over the popcorn. Gently fold the popcorn using two spoons until most of the popcorn is coated in the marshmallow mixture.
Pour the popcorn onto the baking sheet. Spread the mixture out as best you can until the sheet is covered.
Sprinkle the Christmas sprinkles evenly over the popcorn. Then sprinkle the M&M's on top.
Enjoy it right away, or wait for it to cool. After about an hour it becomes less sticky and comes apart more easily.
1. I used salted pre-popped plain popcorn from the store, but if you're popping your own, you'll need approximately 1/4 cup plus 1 Tablespoon of unpopped kernels to make 8 cups of popcorn. Be sure to add salt after you pop it!2. When you're spreading the popcorn onto the baking sheet, spray your hands with cooking spray before handling the popcorn, or wear food handling gloves to prevent sticking.