Preheat the oven to 350º F. Line a 9×13 inch pan with aluminum foil or parchment paper.

In a mixing bowl, combine the flour, salt and cocoa powder. Set aside.

In a small saucepan over low heat, combine the chocolate chips and butter. Stir occasionally until fully melted, about 5 minutes.

In a larger mixing bowl, beat the sugar and eggs plus additional egg yolk with an electric mixer for 1 minute, until the mixture is pale yellow and doubled in volume.

Once the chocolate is melted, pour it into the egg mixture, add the vanilla extract. Mix to combine.

Spoon the flour mixture into the wet ingredients, mixing with a spatula just until combined.

Pour the brownie batter into the prepared pan and make the chocolate chip cookie layer.

In a mixing bowl, combine the flour, cornstarch, baking soda, salt, and 1/2 cup of chocolate chips. Set aside.

In a larger mixing bowl, combine the butter, brown sugar, and granulated sugar. Beat on medium speed for 1 minute. You want this mixture to be fluffy and pale in color.

Add in the egg and vanilla extract, beat for 30 seconds.

Gradually mix in the dry ingredients, mixing on low speed. The dough will be thick.

Drop dollops of cookie dough evenly over the brownie batter. You want cookie dough balls surrounded by brownie batter, maybe a little swirled, but not fully swirled together. The brownie batter will rise up and surround the cookie dough when baking. Press the remaining 1/2 cup chocolate chips onto the top.

Bake at 350ºF for 20 minutes, then cover the pan with aluminum foil and bake for an additional 15-25 minutes. The foil will help prevent the cookie dough from becoming too brown.