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

In a heatproof bowl, combine the chopped dark chocolate and butter. Place over a pan of simmering water (double boiler), ensuring the bowl doesn’t touch the water. Stir until melted and smooth. Remove from heat and stir in the vanilla.

In a separate bowl, use an electric mixer or whisk on high to beat the eggs, caster sugar, and Demerara sugar until the mixture becomes very light, thick, and fluffy (about 5 minutes).

In another bowl, sift together the flour, cocoa powder, baking powder, and salt. Stir in the chocolate chips.

Gently fold the melted chocolate and butter mixture into the whipped eggs and sugar, taking care to keep as much air in the batter as possible.

Carefully fold the dry ingredients into the wet mixture with a spatula until just combined — do not overmix.

Drop rounded tablespoons of batter onto the prepared tray, spacing them about 3–4 cm apart.

Bake for 12–14 minutes until the tops are set and crackly but the centers remain soft.

Remove from the oven and, if desired, sprinkle with a little flaky sea salt. Let cool on the tray for about 10 minutes, then transfer to a cooling rack to cool completely.