Preheat the oven to 350°F (175°C) and line a 9x9-inch baking pan with parchment paper.Prepare the brownie batter by mixing the melted butter, sugar, eggs, vanilla, cocoa powder, flour, salt, and baking powder.Spread the brownie batter evenly into the prepared pan.In a separate bowl, prepare the cookie dough by creaming the butter and sugars, then mixing in the egg and vanilla.Stir in the flour, baking soda, salt, and chocolate chips.Drop spoonfuls of cookie dough evenly over the brownie layer.Bake for 23–28 minutes, or until the cookie layer is golden and a toothpick inserted into the brownie layer comes out with a few moist crumbs.Cool completely before slicing into bars.