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, Reese's pieces, and chocolate chips with a rubber spatula.
Cover the bowl and place in the fridge for at least 30 minutes.
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, Reese's pieces, and chocolate chips as desired.
Bake the cookies for about 12 minutes.
Allow cookies to cool completely on the baking sheet, top with candy eyes if desired, and enjoy!