Preheat the oven to 450°F (230°C). Grease four ramekins with butter and dust with flour.
Melt the butter and chocolate together in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and let cool slightly.
In another bowl, whisk together the eggs, egg yolks, sugar, and salt until thickened and pale.
Fold the melted chocolate mixture into the egg mixture. Sift the flour over the batter and gently fold it in.
Divide the batter evenly among the prepared ramekins. Place the ramekins on a baking sheet and bake for 12 minutes, until the edges are set but the centers are still soft.
Let the cakes cool in the ramekins for 1 minute, then run a knife around the edges to loosen. Invert onto plates and serve immediately.