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 together the melted butter and granulated sugar until well combined. Add the eggs and vanilla extract, beating vigorously until the mixture is glossy and smooth. Fold in the cocoa powder, flour, and salt, stirring 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 both the brown sugar and granulated sugar using a hand mixer or spatula until light and fluffy. Beat in the egg and vanilla extract until fully incorporated. Add the flour, baking soda, and salt; mix 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 an offset spatula or the back of a spoon 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 fudgy and slightly soft in the middle.
06 - Allow the brookies to cool completely in the pan. Once fully cooled, lift the parchment overhang to remove the slab and cut into 16 even squares using a sharp knife.