Put your butter and marshmallows together in bowl and melt in 30 - 60 second intervals in the microwave. You can also melt them on the stove in a pot.
Stir in Rice Krispies Cereal well.
Stir in M&Ms thoroughly.
Spread your treats into a greased pan and let cool for 20 - 40 minutes.
Cut into squares and then they are ready to eat.