Preheat oven to 350°F (175°C). Grease a 9x9-inch baking pan.
In a bowl, whisk together melted butter, sugar, eggs, and vanilla for the brownie layer. Stir in cocoa powder, flour, salt, and baking powder. Spread into prepared pan.
In another bowl, cream together butter, brown sugar, and granulated sugar for the cookie dough. Beat in egg and vanilla. Mix in flour, baking soda, and salt, then fold in chocolate chips.
Drop spoonfuls of cookie dough over the brownie batter and gently spread without mixing.
Bake for 25-30 minutes or until a toothpick inserted comes out with a few moist crumbs.
Let cool before cutting into squares. Enjoy!
Notes
For extra gooey brookies, underbake slightly and let cool completely before cutting.