Preheat your oven to 350°F (175°C).

Melt the butter for the brownie layer in a saucepan over low heat.

In a bowl, mix melted butter, sugar, eggs, and vanilla until smooth.

Stir in flour, cocoa powder, salt, and baking powder until just combined.

For the cookie layer, cream together softened butter, granulated sugar and brown sugar until creamy.

Add eggs and vanilla extract to the cookie dough mixture, mixing until well combined.

In a separate bowl, whisk together the flour, baking soda, and salt. Gradually add this dry mixture to the wet mixture, combining until just mixed.

Fold in the chocolate chips until evenly distributed throughout the dough.

Pour the brownie batter into a greased 9x13 inch baking pan, spreading it evenly.

Drop spoonfuls of cookie dough over the brownie layer. Spread the cookie dough gently without mixing it into the brownie batter.

Use a knife to swirl the brownie and cookie layers together for a marbled effect.

Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted comes out with a few moist crumbs.

Allow the Brookies to cool in the pan for about 15 minutes before transferring them to a cooling rack to cool completely.

Serve warm or at room temperature, and enjoy your delicious Brookies!