Preheat your oven to 350F and line a 13x9 pan with parchment paper, using enough paper that it hangs over the sides of the pan (so that you can easily lift the bars out of the pan once they have cooled).
In the bowl of a stand mixer or in a large bowl using an electric hand mixer, cream together butter, peanut butter, and sugars until well-combined.
Add eggs, egg yolk, and vanilla extract, stirring until combined.
In a separate medium-sized bowl, whisk together flour, cornstarch, baking powder, and salt.
Gradually stir dry ingredients into wet until completely combined.
Add milk and stir well.
Spread batter evenly into prepared pan and bake on 350F for 25-28 minutes (toothpick inserted in the center of the bars should come out dry or with a few moist crumbs).