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

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

In a mixing bowl, beat the eggs, caster sugar, and demerara sugar until pale and fluffy, about 5 minutes.

Sift together the plain flour, cocoa powder, baking powder, and salt in a separate bowl. Fold in the chocolate chips.

Carefully fold the melted chocolate mixture into the egg mixture until combined.

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

Drop tablespoons of batter onto the prepared tray, leaving space for spreading. Bake for 12-14 minutes or until the tops are set.

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