Over medium heat,melt butter or margerine in 8 qt. non-stick pot. Add marshmallows and jello mix and stir until smooth and completely melted. Add cereal, M&Ms and cranberrys and quickly mix all ingredients until coated. Press into buttered 9X12 pan and allow to cool. Cut into 2 inch squares. You can also press into heart shaped molds if your fast enough.