Lightly grease an appropriately sized square or rectangle container with nonstick cooking spray.
Melt the butter in a large pot over medium-low heat. Add the marshmallows and cook, stirring constantly, just until marshmallows have melted.
Remove from heat, pour in Rice Krispies cereal and sprinkles. Gently fold mixture to evenly coat.
Scoop the mixture into prepared container. Using a spatula, spread and gently press and mold the mixture into the container. (Depending upon the container you're using, it may not fill the whole container, however the mixture should be about 1 inch thick.)
Let rest at least 30 minutes then cut into squares.