Generously butter 3 (9cm) ramekins and dust bottom and sides thoroughly with sugar, making sure sugar is covering all the butter inside the dish. Tap out the excess sugar. Set aside.
Preheat your oven to 200℃ (400℉).
In a saucepan melt butter over medium-low heat, add in the flour and whisk for about 1 minute until combined. Add in the milk and whisk until the mixture smooths out and thickens up slightly.
Take it off the heat and pour it over the chocolate. Add in salt and vanilla essence, whisk until the chocolate completely melts and combined. Let cool only slightly (it should still be warm), then whisk in the egg yolk.
In a separate bowl, using an electric mixer beat egg whites at medium speed until the mixture is fluffy. Add sugar, 1 tablespoon at a time, beating until whites hold firm peaks and look glossy.
Gently whisk one-third of the egg whites into the chocolate mixture to lighten up the mixture. Then fold in the remaining whites until just combined. Now spoon the mixture into prepared ramekins. Rub your thumb around the inside of the rim to create about a ¼-inch space between the dish and the soufflé mixture. This will help the soufflé to rise straight.
Transfer ramekins into a baking tray and bake until the soufflés are well risen and the tops are touched with brown, about 12 to 15 minutes (bake it a little more for a firmer soufflé). Remove from oven, dust with cocoa powder and serve immediately.