1. Preheat the oven to 425°F (220°C).
2. Grease four ramekins with butter and dust with cocoa powder.
3. In a microwave-safe bowl, melt the dark chocolate and butter together until smooth.
4. In a separate bowl, whisk together the eggs, egg yolks, and sugar until pale and fluffy.
5. Fold the melted chocolate mixture into the egg mixture.
6. Gently fold in the flour, vanilla extract, and salt until just combined.
7. Divide the batter evenly among the prepared ramekins.
8. Bake for 12-14 minutes, until the edges are firm but the center is still soft.
9. Let cool for 1 minute, then invert onto plates and serve immediately.

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