Preheat the oven to 425°F (220°C).
Butter four ramekins and lightly dust with flour. Arrange on a baking sheet.
Melt the butter and chopped dark chocolate in a heatproof bowl until smooth, using a microwave in short bursts or a double boiler.
Whisk powdered sugar into the melted chocolate mixture until combined.
Add eggs and egg yolks one at a time, whisking well after each addition.
Stir in vanilla extract.
Fold in the flour and pinch of salt gently until just combined.
Divide the batter evenly among the prepared ramekins.
Bake for 11 to 13 minutes until edges are firm and centers are soft and slightly jiggly.
Allow cakes to rest for 1 minute after baking.
Invert cakes onto serving plates carefully or serve directly in ramekins.
Optionally dust with powdered sugar or serve with vanilla ice cream and fresh berries.