Preheat the Oven: Set your oven to 160ºC (320ºF) with a fan or 170ºC (338ºF) for conventional heating. Prepare a baking tray by lining it with parchment paper.

Melt Chocolate and Butter: In a heatproof bowl, combine the chopped dark chocolate and butter. Place this bowl over a pan of simmering water, ensuring the bowl does not touch the water. Stir until melted and combined. Stir in vanilla extract.

Whip Eggs and Sugars: Use an electric hand whisk on high speed to beat the caster sugar, demerara sugar, and eggs until very light, thick, and fluffy, about 5 minutes.

Combine Dry Ingredients: In another bowl, sift the flour, cocoa powder, baking powder, and salt, then add chocolate chips.

Integrate Chocolate Mixture: Gradually fold the melted chocolate and butter mixture into the whipped eggs and sugars. Stir gently to maintain airiness.

Fold in Dry Ingredients: Carefully fold the dry ingredients into the wet mixture using a spatula, mixing just until combined.

Bake the Brookies: Scoop tablespoons of batter onto the prepared baking tray, spacing them apart. Bake for 12-14 minutes, until set and crackly.

Cool and Finish: Remove from the oven, sprinkle with sea salt if desired, and let cool on the tray for about 10 minutes before transferring to a cooling rack.