01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk together melted butter, granulated sugar, and brown sugar until combined. Add eggs and vanilla extract, whisking until smooth.
03 - Sift in cocoa powder, flour, and salt. Gently fold until just combined; do not overmix.
04 - Spread brownie batter evenly into the bottom of the prepared pan.
05 - In a separate bowl, cream softened butter, brown sugar, and granulated sugar until light and fluffy. Beat in egg and vanilla extract.
06 - In another bowl, whisk together flour, baking soda, and salt. Gradually add to wet ingredients, mixing until just combined.
07 - Fold in chocolate chips until evenly distributed throughout the cookie dough.
08 - Drop spoonfuls of cookie dough over the brownie batter, gently spreading to cover. It's acceptable if some brownie batter peeks through.
09 - Bake for 33-36 minutes, or until the top is golden and a toothpick inserted in the center comes out with a few moist crumbs.
10 - Cool completely in the pan on a wire rack before lifting out and slicing into bars.