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 well combined. Add the eggs and vanilla extract, whisking vigorously until the mixture is smooth and glossy.
03 - Gently fold in the flour, cocoa powder, and salt using a spatula, stirring just until no dry streaks remain. Spread the brownie batter evenly across the bottom of the prepared pan.
04 - 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.
05 - Whisk together the flour, baking soda, and salt in a small bowl. Gradually add the dry mixture to the wet ingredients, stirring until just combined. Fold in the chocolate chips evenly throughout the dough.
06 - Drop spoonfuls of the cookie dough over the brownie batter. Gently spread with a spatula to cover the surface as evenly as possible — some brownie peeking through is perfectly fine.
07 - Bake for 28 to 33 minutes, until the cookie top is golden and a toothpick inserted into the center comes out with a few moist crumbs attached. Do not overbake.
08 - Let the brookies cool completely in the pan. Use the parchment overhang to lift the slab out, then cut into 16 even squares.