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 smooth and glossy. 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 mixing bowl, cream the softened butter with both brown sugar and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until fully incorporated. 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 flatten and spread the cookie dough using a spoon or offset spatula to mostly cover the brownie layer—it is perfectly 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. Avoid overbaking—the brookies should remain dense and fudgy in the middle.
06 - Allow the brookies to cool completely in the pan before using the parchment overhang to lift them out. Cut into 16 even squares and serve.