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, allowing overhang on sides for easy removal.
• Prepare the brownie mix according to package directions with eggs, water and vegetable oil, then pour into the prepared baking pan spreading evenly.
• In a separate bowl, beat melted butter with light brown sugar and granulated sugar until well combined.
• Add the whole egg, egg yolk, and vanilla extract to the sugar mixture and beat until smooth.
• Mix in the all-purpose flour, cornstarch, baking soda, and salt until no dry streaks remain.
• Fold in the semi-sweet chocolate chips evenly into the cookie dough.
• Spoon small dollops of cookie dough evenly over the brownie batter layer.
• Bake covered with aluminum foil for 20 minutes, then remove the foil and bake an additional 15-20 minutes until a toothpick comes out mostly clean and cookie layer is golden brown.
• Allow the brookie to cool completely in the pan at room temperature.
• Use the parchment paper overhang to lift the brookie out of the pan and cut into 12 large bars to serve.