Preheat your oven to 375°F (190°C).

Chop the bittersweet chocolate and place it in a heatproof bowl along with the butter. Set the bowl over a saucepan of simmering water to create a double boiler. Stir until fully melted and smooth. Remove from heat and let it cool slightly.

In a separate bowl, whisk the egg yolks with the granulated sugar and vanilla extract until the mixture becomes pale and fluffy, about 3-4 minutes.

Gently fold the melted chocolate into the egg yolk mixture until well combined.

In a clean bowl, whip the egg whites with a pinch of salt until stiff peaks form.

Carefully fold one-third of the egg whites into the chocolate mixture to lighten it. Then gently fold in the remaining egg whites until there are no visible streaks.

Grease individual ramekins with butter and dust with granulated sugar to prevent sticking.

Spoon the soufflé batter into the ramekins, filling them about three-quarters full.

Place the ramekins on a baking sheet and bake for 20-25 minutes until they have risen and the tops are set but the centers remain soft.

Dust with powdered sugar and serve hot.