Preheat the oven to 350°F. Line a 9x13-inch baking dish with parchment paper with overhanging sides.
Make the box mix of brownies according to the package directions. Pour into the baking dish and set aside.
In a separate bowl, add the softened butter, sugar, brown sugar, egg, and vanilla extract. Using a hand mixer, beat until the mixture is light and fluffy. Set aside.
In a separate bowl, add the flour, baking soda, and salt. Use a whisk to combine.
Add the dry mixture to the wet ingredients. Use a large spoon to combine. Then add the chocolate chips and mix them together.
Drop spoonfuls of cookie dough over the top of the wet brownie mix.
Bake in the oven for 30 to 35 minutes, or until a toothpick in the center of the brookies comes out clean.
Let cool in the pan for 15 to 20 minutes before serving.
Video
Notes
The overhanging parchment paper will help you to lift out the entire square of bars so that you can cut them into smaller squares easier.
Ovens can vary quite a bit, so always rely on the toothpick testing method to tell you when the brookies are done. Your brookies could take up to 40 minutes to bake, depending on your oven. Check the brookies at 30 minutes, then continue to bake until a toothpick inserted into the center comes out clean.