Prepare the Brownie Layer:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Line a 9×9-inch baking pan with parchment paper or grease it lightly.

2. Melt the Chocolate and Butter: In a heatproof bowl, melt the dark chocolate and butter together. Microwave in short intervals, stirring in between, or melt over a double boiler on the stovetop. Let it cool slightly.

3. Mix the Dry Ingredients: In a small bowl, whisk together flour, cocoa powder, baking powder, and salt.

4. Combine the Wet Ingredients: In a separate large bowl, beat the eggs, caster sugar, and Demerara sugar until smooth and light in color. Stir in the vanilla extract.

5. Add the Chocolate Mixture: Slowly pour the melted chocolate mixture into the wet ingredients, stirring until fully combined.

6. Add the Dry Ingredients: Gradually fold in the dry ingredients, mixing until just combined. Be careful not to overmix to keep the brownies fudgy.

7. Spread in the Pan: Pour the brownie batter into the prepared baking pan and smooth it out evenly.

Prepare the Cookie Layer:

8. Add Chocolate Chips: Stir chocolate chips into the cookie dough. The dough will be thick, so ensure it’s thoroughly mixed.

9. Top the Brownies: Spoon the cookie dough over the brownie batter and spread it evenly. Some brownie batter will peek through, and that’s perfectly fine!

Bake the Brookies:

10. Bake: Bake for 25-30 minutes, or until the top is golden and a toothpick inserted comes out with a few moist crumbs. The brownies should be set but still fudgy, and the cookie top should be slightly crispy.

11. Cool and Slice: Allow the Brookies to cool for 10 minutes in the pan before transferring them to a wire rack to cool completely. Slice into squares or bars.