The key is only mixing each batter long enough to break up any lumps, then layering the cookie dough atop the brownie batter.
Ingredients
1 package brownie mix (and any ingredients required)
1 roll chocolate chip cookie dough (or cookie dough mix)
Instructions
Preheat the oven to the temperature listed on the brownie mix (typically 350 degrees F.)
Prepare the brownie batter according to the package's instructions. Pour it into whichever size baking pan the package requests (13"x9" or 8"x8" are most common).
Rinse out the brownie batter bowl and use it to make the cookie dough. Or, if you're using a premade tube of dough, slice it into ½-inch-thick coins. Flatten out each ball of dough and layer it atop the brownie batter. Don't cover the brownie batter completely.
Bake in the oven for 40-42 minutes, then insert a toothpick to check doneness. If it comes out mostly clean, turn off the oven and let it cool. If not, heat for another 5-7 minutes, or until it passes the toothpick test.
Recipe by Collegiate Cook at https://collegiatecook.com/2014/02/03/best-cookie-brownies/