In a medium saucepan over medium heat, combine the mini marshmallows and butter.
Stir until the mixture is melted, then remove from heat.
Coat a large bowl lightly with cooking spray, add the cereal and marshmallow sauce, stir until the cereal is well coated with the sauce.
Add the cereal mixture to a small casserole dish that has been coated with cooking spray.
Spread with a spatula (one that’s been coated with the spray is helpful since this mixture gets sticky!) until one even layer.
Refrigerate for at least one hour before cutting into treats.