No need to pick between brownies or chocolate chip cookies with this recipe! Enjoy both classic favorites in a single bite with this dessert that layers chocolate chip cookie dough over your favorite brownie box mix. Since cookies and brownies bake at different times, the secret to making this one work perfectly is keeping the pan covered with foil for the first part of the bake time.
Preheat oven to 350°F. Prepare a 9" square baking pan by lining with foil so that it overlaps on two sides. Spray the foil and sides with baking spray.
In a large bowl, whisk together egg, milk, oil, salt, and espresso powder if using. Whisk in brownie mix until smooth (about 2 minutes). Pour batter into prepared pan and spread evenly.
Open cookie dough package and scatter lumps of cookie dough all over the brownie mix. Press in slightly and layer a few more chunks of dough on top. (Note: it's OK if some of the cookie dough is higher — it will sink a bit as it bakes. The idea is for some of it to sink into the brownie and some of it to remain on top.)
Cover pan with foil and bake 20 minutes. Remove foil and bake an additional 10 to 12 minutes or until the brownie batter is mostly set in the center. (It should jiggle just a tiny bit, but not still be liquid.)
Remove from oven and let cool in pan 15 minutes before loosening edges and pulling out using the foil. Place on a rack to cool completely (for best results, chill in refrigerator at least 4 hours before cutting).