Preheat your oven to 180°C (350°F).
Grease four ramekins with butter and dust with flour.
Melt the dark chocolate and butter over low heat, stirring until smooth.
Stir in the sugar until well combined.
Add the eggs one at a time, mixing well after each addition.
Stir in the vanilla extract.
Sift in the flour and salt, mixing until just combined.
Pour the batter into the prepared ramekins.
Bake for 12-15 minutes until the edges are firm but the center is soft.
Let cool for a minute before inverting each ramekin onto a plate.