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 melted butter and granulated sugar until combined. Add eggs and vanilla extract, whisking until smooth and glossy. Fold in flour, cocoa powder, and salt until just incorporated. Spread the batter evenly across the bottom of the prepared pan.
03 - In a separate bowl, beat softened butter, brown sugar, and granulated sugar until light and creamy. Mix in the egg and vanilla extract until well blended. In another bowl, whisk together 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 cookie dough over the brownie batter and gently spread with a spatula to cover the surface. It is fine if some brownie batter peeks through.
05 - Bake for 28 to 33 minutes, or until the cookie layer is golden and a toothpick inserted into the center comes out with a few moist crumbs. Do not overbake.
06 - Cool completely in the pan before lifting out using the parchment overhang. Cut into 16 bars and serve.