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

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

Sift together the 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. Stir gently to maintain airiness.

Carefully fold the dry ingredients into the wet mixture using a spatula, mixing only until just combined to keep the batter light.

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.