Preheat your oven to 160°C (320°F) for a fan oven or 170°C (338°F) for a conventional oven. Line two baking trays with parchment paper.

Melt dark chocolate and butter in a heatproof bowl over simmering water. Stir until smooth, then mix in the vanilla extract.

In a separate bowl, whisk room-temperature eggs with caster and demerara sugars until airy and pale, about 5 minutes.

Sift together plain flour, cocoa powder, baking powder, and salt in another bowl. Stir in chocolate chips if using.

Fold the melted chocolate mixture into the whipped eggs and sugars carefully.

Gently fold the dry ingredients into the chocolate-egg blend until combined.

Drop dollops of batter onto the prepared trays, spacing them out. Bake for 12-14 minutes until tops are crackly.

Cool on trays for about 10 minutes. Optionally sprinkle with sea salt before transferring to a wire rack to cool completely.