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, making sure the bottom of the bowl does not touch the water. Stir occasionally until the chocolate and butter are fully melted and combined. Stir in the vanilla extract.

Whip Eggs and Sugars: In a separate medium bowl, use an electric hand whisk on high speed to beat the caster sugar, demerara sugar, and eggs. Continue whisking until the mixture becomes very light, thick, and fluffy, about 5 minutes.

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

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

Fold in Dry Ingredients: Carefully fold the dry ingredients into the wet mixture using a spatula, ensuring not to deflate the mixture. Mix only until just combined.

Bake the Brookies: Scoop tablespoons of batter onto the prepared baking tray, spacing them well apart to allow for spreading. Bake in the preheated oven for 12-14 minutes, or until the brookies are set and crackly on top.

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