Preparation Steps

• Preheat the oven to 325°F. Line a 9x13 inch metal baking pan with parchment paper and lightly spray with non-stick spray. Set aside.
• Prepare the brownie mix according to the package directions (eggs, water, and vegetable oil). Stir until combined.
• Pour the prepared brownie batter into the baking pan and spread evenly.
• In a separate bowl, beat melted butter with light brown sugar and granulated sugar until fully combined.
• Add the egg, egg yolk, and vanilla extract to the butter and sugar mixture. Beat until smooth.
• Add flour, cornstarch, baking soda, and salt to the wet ingredients and mix on low speed until no dry spots remain.
• Fold in the semi-sweet chocolate chips evenly into the cookie dough batter.
• Using two spoons, drop dollops of cookie dough evenly over the brownie batter layer.
• Bake covered with aluminum foil for 20 minutes. Remove foil and bake for an additional 15-20 minutes until a toothpick comes out with a few moist crumbs and the cookie layer is golden brown.
• Remove from oven and allow to cool completely at room temperature.
• Lift the brookie out of the pan using the parchment overhang and slice into 12 large servings or 24 small servings.