In a large bowl, combine the 2 cups of mini marshmallows and the butter. Heat in microwave in 60-90 second bursts, stirring between, until the marshmallows are melty. Add Lucky Charms™ cereal (NOT the charms). Stir to coat. The mixture shouldn't be too marshmallowy – you just want the marshmallows to create a thin coating for the cereal.