Prepare the ramekins: Preheat oven to 375°F (190°C). Grease two ramekins with butter and dust them lightly with granulated sugar. Set aside.

Melt the chocolate: In a heatproof bowl, melt the dark chocolate and butter together over a double boiler or in the microwave, stirring every 20 seconds until smooth. Let it cool slightly.

Prepare the egg yolks: In a separate bowl, whisk the egg yolks with 1 tablespoon of granulated sugar until pale and slightly thickened. Stir in the melted chocolate mixture and vanilla extract (if using) until fully combined.

Whisk the egg whites: In a clean bowl, beat the egg whites with a pinch of salt until soft peaks form. Gradually add the remaining 1 tablespoon of sugar and continue to beat until stiff peaks form.

Fold the mixture: Gently fold the whipped egg whites into the chocolate mixture in batches, being careful not to deflate the whites. Fold until just combined.

Bake the soufflé: Spoon the soufflé mixture evenly into the prepared ramekins. Smooth the tops with a spatula and run your finger around the inside edge of the ramekins to create a small gap. Bake for 12–15 minutes, or until the soufflés are puffed and slightly cracked on top.

Serve: Dust with powdered sugar and serve immediately, as the soufflé will begin to deflate once removed from the oven. Enjoy warm!