01 - Preheat oven to 350°F. 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 smooth. Add the eggs and vanilla extract, whisking vigorously until the mixture is glossy and well blended. Fold in the flour, cocoa powder, and salt, stirring gently until just combined — do not overmix. Spread the batter evenly across the bottom of the prepared pan.
03 - 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 a separate 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. Use a spatula to gently spread the cookie dough across 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 clinging to it. Avoid overbaking to maintain a fudgy texture.
06 - Allow the brookies to cool completely in the pan on a wire rack. Once fully set, use the parchment overhang to lift the slab out and cut into 16 bars.