Monster Cookies

Preheat the Oven – Adjust the oven rack to the middle position. Heat to 350°F (177ºC). Line two baking sheets with parchment paper, then set aside.
Prepare the Dry Ingredients – In a medium bowl, whisk together the oats, flour, baking soda, and salt. Set aside.
Cream the Butter and Sugars – In a stand mixer or hand mixer fitted with the whisk attachment, add the softened butter, peanut butter, granulated sugar, and brown sugar. Whisk on medium speed (setting 5) until combined, about 60 seconds. Scrape down the sides as needed.
Mix in the Wet Ingredients – Add the egg and vanilla. Whisk on medium speed until light and fluffy, scraping down the bowl halfway through, about 1 minute.
Combine and Add Chocolates – Using a spatula, stir in the flour mixture until just combined. Stir in ¾ cups of the M&M candies and chocolate chips.
Portion the Dough – Scoop out 2 tablespoon portions (about 1 ounce, 30 grams). Arrange the dough at least 2" apart on lined baking sheets, 13 portions per tray.
Bake the Cookies – Bake one tray at a time in the center position until the cookies are lightly golden brown and set on the edges, 10 to 12 minutes. Bake the remaining tray.
Let Them Cool – Immediately press some of the remaining M&M candies onto the surface of the warm cookie. Leave the cookies on the baking sheet to cool for 3 to 5 minutes, then transfer them to a wire rack to cool completely.