Preheat the oven to 350°F (175°C). Grease a 9x13 inch baking pan or line it with parchment paper for easy removal.
Make the brownie layer: In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar until smooth. Add in the eggs and vanilla extract, mixing until well combined.
Sift in the cocoa powder, flour, salt, and baking powder. Stir until just combined and no flour clumps remain. Set aside.
Prepare the cookie layer: In another bowl, cream the softened butter with the brown sugar and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until fully incorporated.
In a separate bowl, mix together the flour, baking soda, and salt. Gradually add this mixture to the wet ingredients until just combined. Fold in the chocolate chips.
Assemble the brookies: Pour the brownie batter into the bottom of the prepared baking pan, spreading it evenly. Then, dollop spoonfuls of the cookie dough over the brownie layer. Use a spatula or knife to gently swirl them together, creating a marbled effect.
Bake: Place the pan in the preheated oven and bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake.
Cool and Cut: Allow the brookies to cool in the pan for about 10 minutes before lifting them out (if using parchment paper). Let them cool completely on a wire rack before cutting into squares.
Notes
Serve warm with vanilla ice cream and chocolate sauce for an indulgent treat.