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

In a heatproof bowl, combine the chopped dark chocolate and butter. Place the bowl over a pan of simmering water, ensuring the bottom doesn’t touch the water, and stir occasionally until melted and combined. Stir in the vanilla extract.

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

In another bowl, sift together the flour, cocoa powder, baking powder, and salt. Add the chocolate chips into this mix.

Fold the melted chocolate and butter mixture into the whipped eggs and sugars gently to keep the airiness.

Carefully fold the dry ingredients into the wet mixture until just combined.

Scoop tablespoons of batter onto the baking tray, ensuring they are spaced well apart. Bake for 12-14 minutes until set and crackly on top.

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