Preheat the oven to 425°F (220°C). Generously butter four 6-ounce ramekins, dust them with cocoa powder, and tap out the excess. Place the ramekins on a baking sheet.
Melt the dark chocolate and butter together in a heatproof bowl set over simmering water, stirring until smooth. Remove from the heat and let cool slightly.
Whisk the eggs, egg yolks, sugar, vanilla, and salt in a large bowl until pale and slightly thickened, about 2 minutes.
Fold the melted chocolate mixture into the egg mixture, then sift in the flour and fold gently just until no dry streaks remain. Do not overmix.
Divide the batter evenly among the prepared ramekins, filling each about three-quarters full.
Bake for 12 minutes, until the edges are set and slightly pulling away from the ramekins but the centers are still soft and jiggle slightly when shaken.
Let the cakes rest for 1 minute. Run a thin knife around each edge, place a serving plate on top, and confidently invert to unmold. Serve immediately, with vanilla ice cream or fresh raspberries if you like.