Spray a 9×13 pan with non-stick cooking spray.
Set aside 1 ½ cups of marshmallows.
In a large microwave-safe bowl heat butter and remainder of the bag of marshmallows on high for 2 minutes, Stir; microwave for one minute more. Stir until smooth.
Add cereal. Stir until well coated. Stir in M&Ms and reserved marshmallows. (Save a handful of M&M’s to sprinkle on top.)
Using a wet hand press the mixture evenly into the prepared pan. Scatter sprinkles on top and gently press into top. Cool completely before cutting in to 2-inch squares to serve.