Butter inside of 8 x 8 pan and set aside. Melt tablespoon butter in a large saucepan then add marshmallows and stir until melted. Add peanut butter and stir until blended. Add Cheerios and stir as gently and thoroughly as you can. I ended up dumping them into a bowl and using my clean hands. Add M&M's and carefully incorporate (many will break, but the color and flavor will be there and you can save a few to put on the top for presentation). Press into prepared pan (using a piece of wax paper will make this sticky job easier) and cool completely before cutting.