Melt butter and marshmallows over low heat, stirring often.
Add the Rice Krispie treats and stir well.
Add 1 ½ cups of M&M's and stir.
Pour mixture into a 13 x 9-inch pan lined with foil and sprayed with cooking spray.
Press into pan gently using wax paper. Don't press too hard or you will crush the M&M's.
Let cool completely before cutting.