Pop 10 cups of popcorn, salt liberally, and place in a large bowl.
Melt 3 tablespoons of butter in a medium saucepan.
Once the butter is melted, add in 3 cups of mini marshmallows stirring until they are completely melted. Be sure to turn off the heat as soon as the last marshmallow has melted.
Slowly pour around ¾ of the melted marshmallow mixture over the popcorn in the bowl and gently fold it into the popcorn with a spatula or spoon until most of the popcorn is covered.
Pour the marshmallow popcorn onto a prepared cookie sheet, spreading it into a single layer with the spatula.
Drizzle the remaining marshmallow mixture over the popcorn.
Quickly sprinkle the candy and sprinkles over the popcorn while the marshmallow coating is still warm.
Allow the popcorn to cool before breaking into pieces and serving.