01 - Preheat oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two 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, whisking until the mixture is smooth and glossy. Fold in the flour, cocoa powder, and salt until just combined — do not overmix. Spread the batter evenly across the bottom of the prepared pan.
03 - In a separate bowl, beat the softened butter, brown sugar, and granulated sugar together until light and creamy. Mix in the egg and vanilla extract until fully incorporated. In another bowl, whisk together the flour, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined. Fold in the chocolate chips.
04 - Drop spoonfuls of the cookie dough evenly over the brownie batter. Gently spread with a spatula to cover the surface — it is fine if some brownie batter peeks through.
05 - Bake for 28 to 33 minutes, or until the cookie layer is golden and a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to maintain a fudgy texture.
06 - Allow the brookies to cool completely in the pan. Use the parchment overhang to lift the slab out, then cut into 16 bars.