Heat oven to 350 degrees F. In bowl of electric stand mixer or in large mixing bowl with electric hand mixer, cream butter and sugar on medium speed 3 minutes until pale and fluffy. Beat in vanilla, then egg. Beat in peanut butter until just combined.

In separate large bowl, whisk or sift together flour, cocoa powder and baking soda. Add flour mixture to butter mixture and stir on low speed until just combined. Stir in oats, M&Ms and chocolate chips just until combined. Cover bowl with plastic wrap; refrigerate dough 30 minutes to make it easier to shape.

Line a large cookie sheet with parchment paper. Scoop and roll dough into 24-26 balls; place 2 inches apart on cookie sheet. Flatten each ball slightly (dough won't spread that much so this helps them bake into a classic cookie shape). Gently press a few more M&Ms into tops of cookies, if desired. Bake 10-12 minutes. Let cookies cool 5 minutes on cookie sheet before transferring to cooling rack to cool completely.