Preheat oven to 350°F (175°C). Line an 8×8-inch pan with parchment paper, leaving an overhang for easy lifting.
Make the Brownie Batter: In a bowl, whisk melted butter with granulated and brown sugars. Mix in eggs and vanilla. Stir in cocoa powder, flour, and salt until just combined. Fold in chocolate chips. Spread evenly into prepared pan.
Make the Cookie Dough: In a separate bowl, cream softened butter with brown and granulated sugars until light and fluffy. Beat in egg and vanilla. Add flour, baking soda, and salt. Fold in chocolate chips.
Drop spoonfuls of cookie dough over brownie layer and gently spread (it doesn’t need to fully cover the brownie layer).
Bake for 30–35 minutes or until top is golden and a toothpick comes out with moist crumbs.
Cool completely in the pan before slicing into bars.
Notes
Let the brookies cool completely in the pan for the best texture and easiest slicing.