Pre-measure all ingredients before you begin cooking and line a 9×13-inch baking sheet with parchment paper and coat with non-stick spray; set aside. In a large saucepan, melt the butter, salt, and 8 cups of marshmallows together until smooth.
Top with extra M&M’s and sprinkles, if desired, and use parchment paper or greased hands to gently press the mixture into the pan.