Preheat oven to 325°F (163°C). Line a large baking sheet with parchment paper or a silicone baking mat.

Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter, granulated sugar, and brown sugar together in a large bowl on medium-high speed until combined and creamy. Add beaten egg, vanilla extract, and peanut butter then beat on medium-high speed until combined. Scrape down the sides and up the bottom of the bowl as needed. Beat in the flour, oats, baking soda, and salt on low speed, then beat in the M&Ms and chocolate chips until combined. The dough will be sticky. If it’s too sticky to handle, chill in the refrigerator for 5-10 minutes.

Shape into a mound in the center of the lined baking sheet. Bake for 20-30 minutes until edges and top are lightly browned. The center will look super soft, but it will set as the cookie cools. If the cookie isn’t spreading as it bakes, remove it from the oven, then gently flatten with a spoon.

Cool the cookie on the baking sheet for 10 minutes before eating. As it cools, press a few chocolate chips into the top, if desired, for garnish. Cookie freezes well up to 3 months. Thaw at room temperature.