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 vanilla extract.

In a separate bowl, whisk caster sugar, demerara sugar, and eggs together until light, thick, and fluffy, about 5 minutes.

Sift together flour, cocoa powder, baking powder, and salt, then add chocolate chips.

Fold the melted chocolate mixture into the whipped eggs and sugars, stirring gently.

Fold in the dry ingredients carefully with a spatula until just combined.

Scoop tablespoons of batter onto the prepared baking tray, spacing well for spreading.

Bake for 12-14 minutes, or until the tops are set and crackly.

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