Preheat oven to 350 degrees. In a stand mixer with paddle attachment, cream the butter and sugars together on medium speed. Mix in the peanut butter, egg, and vanilla. Slowly mix in the baking soda and flour. Do not overmix. Fold in the M&Ms with a wooden spoon.

Drop dough by tablespoonfuls onto ungreased cookie sheet or silpat. If you want perfect looking cookies, roll a ball on dough in your hands and press down onto your cookie sheet. I pressed a few M&Ms on top of the dough. Bake for 8-9 minutes. Cookies will be soft and may appear undone. They will firm up as they cool.

Allow to cool completely on a wire rack. Enjoy!

Recipe adapted from Sally's Baking Addiction