Melt butter and marshmallows together. Do this in the microwave checking every 30 seconds or so. Or you can do this on low to medium heat in a pan on the stove top.
Once melted and mixed, take off heat and add in the Lucky Charms cereal.
Thoroughly mix marshmallows and Lucky Charms together.
Grease spatula and pan and spread your mixture evenly into the pan.
Let bars sit for 20 - 40 minutes to cool, then enjoy.