Preheat the oven to 325°F. Line a 9x13 inch baking pan with parchment paper and lightly spray with non-stick spray. This helps in easy removal after baking. Set aside.

In a large bowl, prepare the brownie batter by mixing the brownie mix, eggs, water, and oil according to package instructions.

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

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

Add the egg, egg yolk, and vanilla to the wet mixture. Beat until smooth.

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

Fold in chocolate chips evenly into the cookie batter.

Drop spoonfuls of the cookie dough over the brownie layer evenly.

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

Allow to cool completely before lifting out of the pan using the parchment overhang. Slice into 24 bars to serve.