Preheat your oven to 180°C (or 160°C for a fan oven) and line your baking trays with parchment paper.

Melt the dark chocolate and unsalted butter together in a heatproof bowl over simmering water until smooth. Set aside to cool slightly.

In a mixing bowl, beat the large eggs with both sugars and the vanilla extract until light and fluffy, about 5 minutes.

Pour the melted chocolate mixture into the egg mixture and fold gently, then mix in the sifted dry ingredients until just combined.

Scoop portions of batter onto the prepared trays and sprinkle with flaked sea salt, then bake for 12 minutes, until glossy on top.

Let cool on the tray for at least 20 minutes before transferring to a wire rack.