Preheat your oven to 160°C (320°F) for a fan oven or 170°C (338°F) for a conventional oven.

In a heatproof bowl, combine chopped dark chocolate and butter over a double boiler. Stir until smooth.

In a separate bowl, beat eggs with caster and demerara sugars using an electric mixer until light and fluffy (about 5 minutes).

Sift together plain flour, cocoa powder, baking powder, and salt, then fold in chocolate chips.

Gently fold the melted chocolate mixture into the whipped egg and sugar mixture.

Drop rounded scoops of dough onto a lined baking tray. Bake for 12-14 minutes until tops are set.

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