Start by preparing a large mixing bowl. In it, cream together the butter and brown sugar for about 4 minutes. You want the mixture to become creamy and pale, indicating it's well aerated.
Add in the vanilla and mix again until fully incorporated. Next, crack in the egg and beat until smooth; this will add richness to your cookie dough.
Now, it’s time to add the creamy peanut butter. Beat this into the mixture until everything is well combined. The peanut butter will provide that signature flavor!
In a separate bowl, sift together the all-purpose flour, baking soda, and cornstarch. This helps to evenly distribute the leavening agents.
Gently whisk the dry ingredients together, then add them to the wet mixture. Use a spatula to fold the dry ingredients into the wet mixture. Be careful not to overmix; you just want to combine everything.
Once combined, fold in the rolled oats, M&Ms, and chocolate chips. Stir until you see the colorful candies and chocolate distributed evenly throughout the dough.
Cover the bowl with plastic wrap and refrigerate the dough for at least one hour. This step is crucial for developing the flavors and ensuring the cookies hold their shape while baking.
Preheat your oven to 350°F (175°C) and prepare two baking trays by lining them with baking paper. This will prevent the cookies from sticking.
Once chilled, take the cookie dough from the fridge. It should be firm enough for you to roll into balls using your hands. Aim for about 1 heaped tablespoon per cookie.
Place the rolled cookie dough balls onto the prepared trays, leaving some space between them as they won’t spread much. Optionally, you can press a few extra M&Ms and chocolate chips on top for presentation.
Bake the cookies for approximately 12 minutes or until golden brown on the edges. Remove from the oven and allow them to cool slightly before transferring to a wire rack to cool completely. Enjoy!
