In a large bowl, cream together the butter and peanut butter until smooth, about 3 minutes. Add the brown sugar, flour, vanilla and quick oats and beat until combined, about 1 minute. Scrape down the sides of the bowl and add 1 cup of the powdered sugar. Beat well and gradually add in the recipe until well combined. Add more powdered sugar if necessary to reach the desired consistency. Pouw in the M&Ms and stir together using a rubber spatula. Eat immediately or store covered in the freezer for up to 3 months.