Preheat the oven to 375º. Butter four ramekins, rims included, and set aside.

Heat 1 inch of water in a saucepan until it boils, and then turn off the heat. Place the chocolate in a heatproof bowl, and set it on top of the pan (not touching the water). Stir until melted.

In a large bowl, whisk the yolks and half of the sugar together until thick and frothy. Add the vanilla and the melted chocolate.

In a dry clean bowl, whip the egg whites until foamy. Sprinkle in the remaining sugar, and continue to whip until soft peaks form.

Fold the whipped whites into the chocolate mixture one third at a time, until fully incorporated.

Fill the buttered ramekins 3/4 full, and bake until risen but still slightly jiggly, 8 to 10 minutes.

Serve at once dusted with powdered sugar.