Thoroughly grease a 9- by 13-inch pan with softened butter.
In a large pot, melt the butter. Stir in the regular marshmallows until they're totally melted. Stir in the salt. Remove from the heat and set aside.
Pour in the cereal, folding gently the whole time until all combined. Then pour in the mini marshmallows and gently fold in. Turn into the buttered pan and press flat. Immediately top with rainbow sprinkles.
Let cool completely, and then cut into squares.