01 - Preheat oven to 350°F. Line a 9x9 inch square baking pan with parchment paper, leaving a slight overhang on the sides for easy removal.
02 - In a medium bowl, whisk melted butter and granulated sugar until well combined. Add eggs and vanilla extract, beating until the mixture turns glossy and smooth. Fold in 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 softened butter with brown sugar and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until fully combined. Add flour, baking soda, and salt, mixing until just incorporated. Gently fold in the semi-sweet chocolate chips.
04 - Drop spoonfuls of 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 to maintain a fudgy texture.
06 - Allow to cool completely in the pan before lifting out using the parchment overhang. Cut into 16 even squares and serve.