Melt butter in a large saucepan over low heat and add marshmallows.

Stir until they begin to melt, adding in (dry) cake mix one spoonful at a time so its combined.

Stir in cereal so it is completely coated with marshmallow mixture.

Sprinkle in half of the sprinkles and mix. Press into a baking dish (any size will do) and top with remaining sprinkles.

Let sit for about 30 minutes before cutting.

I find spraying a knife with nonstick spray helps to cut them.