Start by creaming the butter, white sugar, and brown sugar with an electric mixer until smooth. Add the egg and vanilla, then mix again. Gently stir in flour, baking soda, salt, and chocolate chips. Refrigerate until ready to use.
Preheat your oven to 350°F (175°C). Prepare your brownie mix as directed on the package, then pour the batter into your greased 8×8 or 9×9 baking dish.
Scatter cookie dough over the brownie batter, pressing gently so it slightly sinks into the base. Don’t worry if it’s uneven—it will spread as it bakes.
Bake for 35–40 minutes. Start uncovered, then cover with foil if the cookies brown too quickly. Check after 35 minutes for gooey brownies or 40+ minutes for fully baked. Let them rest for 15–20 minutes before cutting.