Preheat oven to 350 degrees F. Line a 9x9 square light metal baking pan with foil, extending the sides of the foil over the edges of the pan. Spray the foil liberally with cooking spray; set aside.

In the bowl of a stand mixer, cream together the butter, brown sugar and white sugar until fluffy, about 1 minute. Add in the egg and egg yolk, mixing well after each addition, followed by the vanilla. Lastly, add in the baking soda, cornstarch, salt and flour and mix until combined. Stir in the chocolate and peanut butter chips.

Divide cookie dough in half. Press half of the cookie dough in an even layer in the bottom of the prepared pan. Top with the chopped peanut butter cups in a single, even layer.

In a medium bowl, whisk together the sweetened condensed milk and creamy peanut butter until combined. Pour/spoon the mixture over the peanut butter cup layer evenly. Lastly, crumble the remaining half of the cookie dough over the condensed milk layer. It is okay if the cookie dough doesn't completely cover the condensed milk layer, but try to get it as evenly covered as possible.

Bake for 30-40 minutes or until a toothpick inserted near the center comes out clean. Cool completely before cutting into bars.

NOTE: To make this even easier, use refrigerated chocolate chip cookie dough! Bring a log of refrigerated chocolate chip cookie dough close to room temperature and work in some peanut butter chips. Follow the directions of the recipe here to make the bars.