01 - Preheat oven to 350°F (175°C). Line an 8×8-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 combined. Fold in the flour, cocoa powder, and salt with a spatula, stirring until just incorporated. 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 blended. 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 as much of the surface as possible — some brownie peeking through is perfectly fine and adds to the rustic look.
05 - Bake for 28 to 33 minutes, or until the cookie layer is golden at the edges and a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to maintain the fudgy texture.
06 - Allow the brookies to cool completely in the pan. Once fully set, use the parchment overhang to lift the slab out and transfer to a cutting board. Cut into 16 even bars.