Heat oven to 350°F. Line a 9-inch square pan with parchment, leaving overhang. Lightly grease.
Whisk melted butter and granulated sugar for the brownie layer until glossy.
Whisk in eggs and vanilla until smooth and slightly thickened.
Fold in cocoa, flour, baking powder, and salt just until no dry streaks remain. Spread in pan.
Beat softened butter with brown and granulated sugars for the cookie layer until light and fluffy.
Mix in the egg and vanilla until combined and creamy.
Stir in flour, baking soda, and salt until a soft dough forms. Fold in chocolate chips.
Scoop cookie dough over the brownie batter in large spoonfuls. Gently spread to mostly cover.
Bake 32–38 minutes, until cookie top is golden and a toothpick in brownie shows moist crumbs.
Cool in pan at least 1 hour. Lift out using parchment and slice into 16 bars.