Preheat oven to 350°F (177°C). Line baking sheet with parchment paper or silicone baking mat.

In a large bowl, using a hand-held mixer or stand mixer with paddle attachment, cream the butter and sugars together on medium speed, about 3 minutes. Mix in the peanut butter, egg, and vanilla (in that order). Scrape down the sides of the bowl as needed. Slowly mix in the baking soda, flour, and salt. Do not over-mix. On low speed, beat in the quick oats, M&Ms, and chocolate chips just until incorporated. If the dough is very soft and unmanageable by hand, chill the dough for 30 minutes before rolling.

Rolls balls of dough, about 2 Tablespoons of dough per ball, onto prepared baking sheet. Press a few extra M&Ms on top for looks, if preferred. Bake for 11-13 minutes—the cookies will still look a little soft, which is recommended. Slightly press down the baked cookies with the back of a spoon, since the cookies only slightly spread in the oven. Allow cookies to cool on baking sheet for 10 minutes before transferring to a wire rack. Cookies will firm up as they cool.

Store cookies covered at room temperature for up to 1 week.