In a medium bowl, whisk the oats, baking soda, and salt together until combined. Set aside.

In a large bowl, whisk the melted butter, brown sugar, eggs, and vanilla together until combined. Whisk in the peanut butter.

Pour the dry ingredients into the bowl with the wet ingredients and mix with a silicone spatula until combined. (Takes some arm muscle!) Fold in the M&Ms and chocolate chips. The dough will be sticky and gooey.

Cover the cookie dough tightly and chill in the refrigerator for at least 30 minutes and up to 3 days.

Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats.

Scoop the dough, a heaping 2 Tablespoons (45g) each, and place the mounds of cookie dough about 3 inches apart on the prepared baking sheets. (A medium cookie scoop helps with the sticky dough, but make sure it is a heaping scoop to get you 2 Tablespoons (45g) of dough.)

Bake until the edges are set and golden brown, about 12–15 minutes. The cookies will still look soft in the centers.

Cool cookies for 10 minutes on the baking sheet. During this time, I like to press a few more chocolate chips or M&M’s into the tops of the warm cookies. (This is optional and only for looks.) Transfer to wire rack to cool completely.

Cover leftover cookies tightly and store at room temperature for up to 1 week.