Add butter and marshmallows to a microwave safe bowl.
Microwave for 1 minute. Stir the ingredients together and microwave in 30-second intervals until the marshmallows are completely melted.
Add rice krispie cereal. Stir until well-coated. Quickly stir in half of the M&M's. Work quickly so the color from the candies does not bleed into the cereal mixture.
Using a rubber spatula coated with butter or wax paper evenly press the melted marshmallow mixture into a 13 x 9 x 2-inch baking dish or pan coated with non-stick cooking spray or lined with parchment paper. Cool.