Preheat your oven to 350°F (175°C) and prepare a 9x9 inch square baking pan by greasing it lightly or lining it with parchment paper.
In a medium mixing bowl, combine the melted butter and granulated sugar, stirring until smooth. Add the eggs one at a time, whisking in the vanilla extract. Gradually incorporate the cocoa powder, flour, salt, and baking powder until just combined. Pour the brownie batter into the prepared pan.
In a separate bowl, beat together the softened butter, brown sugar, and granulated sugar until creamy. Add the egg and vanilla extract, mixing thoroughly. Slowly add the flour, baking soda, and salt until just combined. Fold in the chocolate chips.
Spoon dollops of the cookie dough over the brownie layer and swirl gently with a knife to create a marbled effect.
Bake for 25-30 minutes until the edges are set and the middle is slightly gooey. A toothpick should come out with a few moist crumbs.
Allow to cool in the pan for at least 15 minutes before cutting into squares.
Notes
Serve warm with a sprinkle of powdered sugar and a scoop of vanilla ice cream for an indulgent treat.