Grease a 9x9 inch baking dish with butter or cooking spray.

In a saucepan over low heat, melt the butter. Add the marshmallows and stir until completely melted and smooth.

In a large mixing bowl, combine the Rice Krispies and Lucky Charms. Pour the melted marshmallow mixture over the cereals and stir until evenly coated.

Transfer the mixture to the prepared baking dish. Use a spatula or your hands (greased) to press the mixture evenly into the dish.

Allow the treats to cool completely before cutting into squares.