01 - Preheat oven to 350°F. Line a 9×9-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - In a medium bowl, whisk the melted butter and granulated sugar until well combined. Add the eggs and vanilla extract, beating vigorously until the mixture is glossy and smooth. Fold in the cocoa powder, flour, and salt, stirring just until incorporated. Spread the batter evenly across the bottom of the prepared pan.
03 - In a separate bowl, cream the softened butter with brown sugar and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until fully blended. Add the flour, baking soda, and salt, mixing until just combined. Gently fold in the chocolate chips.
04 - Drop spoonfuls of the cookie dough evenly over the brownie batter. Gently press and spread the cookie dough to cover most of the surface — a little brownie peeking through is perfectly fine and creates a beautiful marbled edge.
05 - Bake for 30 to 35 minutes, until the edges are golden and the center is just set. Avoid overbaking — the brookies should remain dense and fudgy in the middle. A toothpick inserted in the center should come out with moist crumbs, not wet batter.
06 - Let the brookies cool completely in the pan. Once fully cooled, use the parchment overhang to lift the slab out and transfer to a cutting board. Slice into 16 even squares.