Melt your butter over low heat in a large stock pot, and add marshmallows and stir occasionally until melted. Then add the rice cereal and gently fold it into the marshmallows until coated.
Pour your cereal and marshmallow mixture into a pan and allow to cool for a few minutes. Flatten it out and cut it into circles using a biscuit cutter. Embellish your circles with M&M's and allow to cool so they harden up a little bit.