Preheat the oven to 325°F. Line a 9x13 inch baking pan with parchment paper and spray with non-stick spray. Set aside.

In a large bowl, prepare the brownie mix according to package instructions (eggs, water, oil). Stir until well combined.

Pour the brownie batter into the prepared pan and spread evenly.

In a separate bowl, beat melted butter, brown sugar, and granulated sugar until smooth.

Add the egg, egg yolk, and vanilla extract to the sugar mixture and beat until smooth.

Add flour, cornstarch, baking soda, and salt to the wet ingredients. Mix on low until incorporated.

Fold in chocolate chips until evenly distributed.

Drop spoonfuls of cookie dough over the brownie layer, spreading slightly.

Bake for 20 minutes covered with foil, then uncover and bake for an additional 15-20 minutes until a toothpick comes out clean.

Cool completely, then lift from pan using parchment and slice into servings.