Preheat oven to 350° F. Line a straight sided metal 9x13 baking pan with parchment paper and lightly spray with non-stick baker’s spray. Be sure to use a piece of parchment paper that is large enough to allow for some overhang on the sides. This will make it easier to lift the solid baked brookie from the baking pan to slice and serve. Set aside.

To make the brownie layer, add to a large mixing bowl the ingredients called for (eggs, water and vegetable oil) on the box of fudgy brownie mix. Stir to combine according to package directions.

Pour the boxed fudge brownie batter into the prepared baking pan, spread to an even layer and set aside.

To make the cookie layer, add to a separate large mixing bowl the light brown sugar, granulated sugar and unsalted butter. Using a handheld mixer on medium speed, beat the sugars and melted butter until fully combined.

Add the whole egg, egg yolk and vanilla extract. Beat on medium speed again to combine all the ingredients until you get a smooth mixture.

To the bowl of wet ingredients, add the all-purpose flour, cornstarch, baking soda and salt. Mix on low speed until no dry streaks of flour remain.

Add the semi-sweet chocolate chips to the cookie batter and, using a rubber spatula or wooden spoon, fold them into the batter until they are evenly distributed.

Using your hands, carefully drop small dollops of the cookie dough evenly over the surface of the brownie batter. You want to make sure that you distribute the cookie dough as evenly as possible over the brownie batter to ensure even baking of both the brownie and cookie layers of the brookie.

Bake the brookie for 30-35 minutes or until a toothpick inserted into the center comes out clean or with just a few moist brownie crumbs on it. Your cookie layer should be golden brown and fully set in the center. If needed, you can tent your brookie with a piece of aluminum foil the last 5-10 minutes of baking to keep the cookie layer from browning too much.

Remove the brookie from the oven and allow it to cool completely on the counter, at room temperature.

When the brookie has cooled completely, use the overhanging parchment paper to carefully lift it out of the pan. Place the whole brookie onto a large cutting board and slice into 12 large, or 24 small, servings.