01 - Preheat oven to 350°F (175°C). Line a 9×9-inch square baking pan with parchment paper, leaving a slight overhang on the 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 until the mixture turns glossy and smooth. Fold in the cocoa powder, flour, and salt until just 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 combined. Add the flour, baking soda, and salt, mixing until just incorporated. Gently fold in the chocolate chips.
04 - Drop spoonfuls of cookie dough evenly over the brownie batter. Gently flatten and spread the cookie dough to mostly cover the brownie layer—some brownie batter peeking through is perfectly fine and adds to the marbled appearance.
05 - Bake for 30 to 35 minutes until the edges are golden and the center is just set. Avoid overbaking to maintain the signature fudgy texture.
06 - Allow the brookies to cool completely in the pan before lifting them out using the parchment overhang. Cut into 16 even squares and serve.