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