01 - Preheat oven to 350°F. 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 is glossy and smooth. Stir in the cocoa powder, flour, and salt until just combined — do not overmix. 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 incorporated. Add the flour, baking soda, and salt, mixing until just combined. Fold in the semi-sweet chocolate chips.
04 - Drop spoonfuls of the cookie dough over the brownie batter in the pan. Gently flatten and spread the cookie dough to mostly cover the brownie layer — it is fine if some brownie batter peeks through.
05 - Bake for 30 to 35 minutes, until the edges are golden and the center is just set. Do not overbake — brookies should remain fudgy in the middle.
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.