Preheat oven to 350°. Set a full kettle of water on to boil.
Butter six 4-ounce soufflé cups and coat with sugar; set aside.
Bring milk to boil in small saucepan. Meanwhile, melt butter in medium saucepan over medium heat. Add the flour and cocoa to butter and heat with a whisk until it has a smooth, paste-like consistency. Reduce heat and cook for 1 minute.
Slowly add hot milk, whisking until smooth, then blend in melted chocolate. Let cool for 5 minutes, then stir in egg yolks.
Beat egg whites in separate bowl until frothy. Slowly add sugar, 1 tablespoon at a time until glossy peaks form. Stir a heaping spoonful of egg whites into the chocolate, then fold in remaining whites until just combined.
Pour the batter into prepared soufflé cups. Place cups in large baking dish and add enough boiling water to reach halfway up the sides of the soufflé cups.
Bake for 20 minutes. Serve immediately with warm Vanilla Sauce.