Prepare the Brownie Base:
Preheat your oven to 350°F (175°C). Grease an 8×8-inch baking dish or line it with parchment paper.
In a mixing bowl, combine the brownie mix, melted butter, water, and egg. Stir until smooth.
Pour the brownie batter into the prepared baking dish, spreading it evenly to cover the bottom.
Make the Cookie Dough:
In a separate bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the egg and vanilla extract, and mix until combined.
Gradually add the flour, baking soda, and salt. Mix until just incorporated.
Fold in the chocolate chips.
Assemble the Brownies:
Scatter pieces of cookie dough over the brownie batter. Gently press the dough down into the batter.
Bake:
Bake for 35-40 minutes. If the top starts to brown too quickly, cover with foil for the first 20 minutes, then uncover for the rest of the baking time.
Let the brownies cool for 20 minutes before cutting them into squares.