Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan.
For the brownie layer: In a mixing bowl, combine melted butter, sugar, and brown sugar. Add eggs and vanilla extract, mix well. Stir in flour, cocoa powder, salt, and baking powder until combined. Spread the brownie batter evenly in the prepared pan.
For the cookie layer: In another mixing bowl, cream together softened butter, brown sugar, and sugar until light and fluffy. Beat in eggs and vanilla extract. Gradually add flour, baking soda, and salt, mixing until just combined. Fold in chocolate chips. Drop spoonfuls of cookie dough over the brownie batter and spread evenly.
Bake in preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely before cutting into bars.