In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, cream together the butter and peanut butter on medium speed for 2-3 minutes.
Add the sugars and continue beating for 2-3 more minutes until light & fluffy.
Beat in the egg, egg yolk, and vanilla until fully combined.
Add the dry ingredients and mix on low speed until just combined.
Lastly, fold in the oats, m&ms, and chocolate chips with a rubber spatula.
Cover the bowl and place in the fridge for at least one hour or overnight.
Once the dough has chilled, preheat the oven to 350 degrees F (180 degrees C) and line two baking sheets with parchment paper.
Use a 3 tablespoon cookie scoop to portion the cookie dough onto a small parchment lined baking sheet.
Freeze the cookie dough for 15 minutes.
Place the cookie dough on the prepared baking sheets about 2 inches a part, and top with more oats, m&ms, and chocolate chips as desired.
Bake the cookies for about 12 minutes.
Allow cookies to cool completely on the baking sheet and enjoy!