Preheat oven to 425°F. Generously butter four 6-ounce ramekins and place them on a baking sheet.

Melt the chocolate and 1/2 cup butter together in a microwave-safe bowl in 30-second intervals, stirring between each, until smooth. Set aside to cool slightly.

In a separate bowl, whisk together the eggs, egg yolks, and sugar until thick and pale, about 2-3 minutes.

Gently fold the melted chocolate mixture into the egg mixture, then add flour, salt, and vanilla extract. Stir until just combined.

Divide the batter evenly among the prepared ramekins, filling each about three-quarters full.

Bake for 11-13 minutes, until the edges are firm but the center still jiggles slightly when gently shaken.

Let cool for 1 minute, then run a knife around the edges and invert onto serving plates. Dust with powdered sugar and serve immediately.