Preheat the 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. Place over a pan of simmering water, stirring occasionally until melted and combined. Stir in the vanilla extract.

In a separate bowl, beat the caster sugar, demerara sugar, and eggs with an electric whisk until light, thick, and fluffy, about 5 minutes.

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

Gradually fold the melted chocolate and butter mixture into the whipped eggs and sugars.

Fold the dry ingredients into the wet mixture using a spatula, mixing only until just combined.

Scoop tablespoons of batter onto the prepared baking tray, spaced apart to allow for spreading.

Bake in the preheated oven for 12-14 minutes, or until set and crackly on top.

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