In a medium saucepan over low heat, melt the salted butter and then mix in marshmallows until glossy and smooth (about 5 minutes).

Remove the saucepan from heat and stir in the dry cake mix until well blended.

In a large mixing bowl, combine the crispy rice cereal with the warm marshmallow mixture, folding gently. Add ½ to ¾ of the sprinkles.

Prepare the baking dish by lightly buttering it, then transfer the mixture and press down firmly and evenly.

Sprinkle the remaining rainbow sprinkles on top, then let the treats cool at room temperature for about 30 minutes.