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 over a pan of simmering water. Stir until melted and combined, then stir in vanilla extract.

Whip Eggs and Sugars: In a separate bowl, beat the caster sugar, demerara sugar, and eggs with an electric whisk until 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, stirring gently.

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

Bake the Brookies: Scoop tablespoons of batter onto the prepared baking tray and bake for 12-14 minutes, until set and crackly on top.

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