Preheat the oven to 350°F (175°C) and grease a 9×13-inch baking pan.
Prepare the brownie batter: Melt the butter and cocoa powder together. Stir in sugar, eggs, and flour. Mix until combined, then pour into the greased pan.
Prepare the cookie dough: Cream the butter, brown sugar, and granulated sugar together until light and fluffy. Add the egg and vanilla extract, then mix in flour, baking soda, and salt. Fold in the chocolate chips.
Drop spoonfuls of cookie dough over the brownie batter, gently pressing down but not spreading completely.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Let the brookies cool completely before slicing and serving.
Notes
For a fudgier brownie base, reduce the flour slightly. Store brookies in an airtight container for up to 3 days, or freeze for up to 3 months.