Preheat Oven and Prepare Pan
Preheat your oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper or lightly grease it.

Mix Dry Ingredients
In a medium bowl, whisk together all-purpose flour, cake flour, cornstarch, baking soda, baking powder, and salt.

Cut in Butter and Sugar
Using a pastry cutter or your fingers, cut the cold butter into the dry ingredients until the mixture resembles coarse crumbs. Add brown sugar and granulated sugar, mixing to combine.

Add Eggs and Vanilla
In a separate bowl, whisk the egg, egg yolk, and vanilla extract. Gradually mix into the butter-flour mixture until just combined.

Fold in Chopped Candies and Oreos
Gently fold in the chopped Hershey’s Cookies ‘n’ Crème Bars, crushed Oreos (reserve 5 Oreos for topping), and mini chocolate chips.

Transfer to Pan and Add Toppings
Spread the batter evenly into the prepared pan. Top with the remaining 5 Oreo cookies, broken into pieces, pressing lightly into the batter.

Bake
Bake for 30-35 minutes or until the edges are golden and a toothpick inserted in the center comes out mostly clean with a few moist crumbs.

Cool and Cut
Allow bars to cool completely in the pan before lifting out and cutting into squares.