01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving enough overhang on two sides to lift the finished brookies out cleanly.
02 - In a medium bowl, whisk the melted butter and granulated sugar until well blended. Add the eggs and vanilla extract, whisking vigorously until the mixture is smooth and glossy. Gently fold in the flour, cocoa powder, and salt with a spatula, stirring only until no dry streaks remain. 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 using an electric mixer until light and creamy, about 2 minutes. Mix in the egg and vanilla extract until fully incorporated. In a small bowl, whisk together the flour, baking soda, and salt. Gradually add the dry mixture to the wet ingredients, stirring until just combined. Fold in the chocolate chips.
04 - Drop spoonfuls of the cookie dough evenly over the brownie batter. Using an offset spatula, gently spread the cookie dough toward the edges. It is fine if some brownie batter peeks through — this creates the signature swirled look.
05 - Bake on the center rack for 28 to 33 minutes. The cookie layer should be lightly golden and a toothpick inserted into the center should come out with a few moist crumbs clinging to it. Avoid overbaking to maintain a fudgy texture.
06 - Let the brookies cool completely in the pan on a wire rack. Once fully set, use the parchment overhang to lift the entire slab out. Cut into 16 even squares using a sharp knife.