01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, allowing overhang on sides for easy removal.
02 - In a large bowl, whisk melted butter, granulated sugar, and brown sugar until combined. Add eggs and vanilla extract, whisking until smooth.
03 - Sift cocoa powder, flour, and salt into the wet mixture. Gently fold until just combined; avoid overmixing.
04 - Evenly spread brownie batter across the bottom of the prepared pan.
05 - In a separate bowl, beat softened butter, brown sugar, and granulated sugar until light and fluffy. Mix in egg and vanilla extract until incorporated.
06 - Whisk flour, baking soda, and salt in another bowl. Gradually add to the wet mixture, mixing until just combined.
07 - Fold semisweet chocolate chips into the cookie dough.
08 - Drop spoonfuls of cookie dough over the brownie layer, gently spreading to cover. Small gaps where brownie shows through are acceptable.
09 - Bake for 33-36 minutes until golden on top. A toothpick inserted in the center should emerge with a few moist crumbs.
10 - Cool completely in the pan on a wire rack. Lift out using parchment overhang and cut into bars.