Preheat oven to 350°F (180°C), or 160°C if using a fan oven. Lightly grease a 9×13-inch (approx 22x33 cm) baking dish, line it with parchment paper if desired, and set it aside.
Cookie Dough:
In the bowl of a stand mixer, equipped with a paddle attachment, cream the butter, granulated sugar, and brown sugar together until light and fluffy.
Add the eggs and vanilla and mix until just combined. Then add the flour, cornstarch, and baking soda. Mix until a dough forms.
Lastly, fold in the chocolate chips until they are evenly distributed throughout the dough.
Press the cookie dough into an even layer on the bottom of the prepared pan.
Top the cookie dough layer with the Oreo cookies, gently pressing them into the cookie dough.
Brownie Batter:
Prepare the brownie layer by melting the chocolate chips and butter in a medium microwave safe bowl on 30 second increments until fully melted and smooth.
Add in the sugar and mix until well combined.
Add in the eggs and vanilla and mix until just combined.
Lastly, add the cocoa powder and flour and fold until no dry spots are remaining in the batter.
Pour the brownie batter over the Oreo layer being sure to cover each cookie. Smooth evenly.
Bake for 35-40 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.
Remove from the oven and allow to cool completely before cutting.
If desired, drizzle melted chocolate and top with crushed Oreo cookies for topping.
Notes
Prepare your brownie pan. Be sure to grease the pan well with cooking spray. For extra ease in removing the brookies once they've cooled, I suggest adding a layer of parchment paper as well.
Try different cookies. Regular Oreos are classic, but Nabisco makes all sorts of tasty Oreo flavors that you can add to this recipe. Try making brookies with Mint Oreo, Chocolate Oreo, or Peanut Butter Oreo!
Don't overmix. This is true for both the chocolate chip cookie dough and the brownie batter. Only mix in the flour until you can't see it anymore. The brownie batter may look a little bit lumpy, and that's totally ok.
Let them cool. For neatly sliced brookie squares, it's important to let the bars cool down completely before slicing them. You can chill the pan briefly in the refrigerator to make cutting even easier.
To Store: Keep the bars in an airtight container at room temperature for up to 3 days.