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

In a heatproof bowl, combine the chopped dark chocolate and butter, then place this bowl over a pan of simmering water. Stir occasionally until fully melted and combined. Stir in the vanilla extract.

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

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

Gradually fold the melted chocolate and butter mixture into the whipped eggs and sugars, stirring gently to maintain airiness.

Carefully fold the dry ingredients into the wet mixture using a spatula, ensuring not to deflate the batter. Mix only until just combined.

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

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