01 - Preheat the 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 the melted butter and granulated sugar until well combined. Add the eggs and vanilla extract, beating until the mixture is 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 the 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 the cookie dough over the brownie batter. Gently flatten and spread the cookie dough to mostly cover the brownie layer—it is perfectly fine if a little 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—the brookies should remain fudgy in the middle.
06 - Allow the brookies to cool completely in the pan. Use the parchment overhang to lift the slab out, then cut into 16 squares using a sharp knife.