Over LOW heat, melt the butter in a large dutch oven or large stockpot. Once the butter is fully melted, add in the 10-ounce bag of mini marshmallows. Stirring constantly (I use a rubber spatula) until the marshmallows are melted. Once they are melted and the mixture is smooth, add in the large marshmallows, and stir. Let them semi-melt. Remove the pan from the heat. Add the pinch of salt and then add in the cereal and sprinkles if using. Stirring quickly to make sure that all is combined. Transfer to prepared baking dish. I spray a little nonstick spray on my hands and then lightly press the mixture down. Let them cool at room temperature for at least an hour.