Weigh out the ingredients
You'll need flour, butter, peanut butter, brown sugar, granulated sugar, eggs, oats, Reese's pieces, chocolate chips, vanilla, cornstarch, baking soda, baking powder, and salt.
Combine dry ingredients
In a medium or large mixing bowl, stir 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.
Cream butter & peanut butter
Beat in the egg, egg yolk, and vanilla until fully combined.
Add eggs and vanilla
Add the dry ingredients and mix on low speed until just combined.
Add dry ingredients
Lastly, fold in the oats, Reeses pieces, and chocolate chips with a rubber spatula. Cover the bowl with plastic wrap and place in the refrigerator for at least one hour or overnight.
Fold & chill
Press the dough into the pan and top with more Reese's pieces and chocolate chips. Bake the monster cookie bars for about 22 minutes or until evenly golden. Allow to cool completely in the pan before removing.