Preheat your oven to 350°.
In a mixing bowl, cream together the softened butter, brown sugar, and granulated sugar for about 3 minutes or until light and fluffy.
Add the eggs and vanilla extract, mixing until well combined. Fold in the creamy peanut butter.
In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, cornstarch, and salt. Gradually add the dry ingredients to the wet mixture, mixing until a dough forms.
Gently fold in the mini chocolate chips.
Line a 9x9inch baking pan with parchment paper, allowing some overhang on the sides for easy removal.
Press the cookie dough evenly into the prepared pan, creating a smooth and level surface.
Bake in the preheated oven for 30-35 minutes or until the top is golden brown and a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake.
Allow the bars to cool in the pan for about 20 minutes. Then, using the parchment paper overhangs, carefully lift the bars out of the pan and transfer to a wire rack to cool completely.
While the bars are cooling, prepare the topping. In a microwave safe dish, melt the peanut butter in 20 second increments, stirring between each interval, until smooth.
Once the bars have cooled completely, drizzle the melted peanut butter over the top and sprinkle with chopped peanut butter cups.
Enjoy your delicious peanut butter chocolate chip cookie bars!