Preheat your oven to 200°C (392°F).
Grease four ramekins with butter and dust with flour.
Melt the dark chocolate and butter in a bowl over simmering water.
In a separate bowl, whisk together the eggs and sugar until frothy.
Gradually add the melted chocolate mixture to the egg mixture, stirring constantly.
Sift the flour and salt into the chocolate mixture and fold gently.
Pour the batter into the prepared ramekins, filling them about ¾ full.
Place the ramekins on a baking tray and bake for 10 minutes.
Remove from the oven and let them sit for 1 minute.
Carefully invert each ramekin onto a plate and serve immediately.