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

Melt dark chocolate and butter in a heatproof bowl over simmering water, stirring until smooth (about 5 minutes). Stir in vanilla extract and let cool slightly.

Whip together caster sugar, demerara sugar, and room temperature eggs until light and fluffy for about 5 minutes.

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

Combine the melted chocolate mixture with the whipped egg mixture, then fold in the dry ingredients without overmixing.

Dollop the batter onto the prepared tray and bake for 12-14 minutes until edges set and tops crackle.

Cool in the tray for 10 minutes, then transfer to a cooling rack. Optionally sprinkle sea salt on top.