Put both chocolates in a metal mixing bowl and warm over steam.
As soon as they are liquid, stir in the butter, remove from heat, and mix in the egg yolks.
Beat the egg whites to stiff peaks with the sugar and fold into the chocolate mixture.
Butter a ramekin and sprinkle with granulated sugar.
Fill the ramekin about 2/5 of the way full and bake in a water bath for about 25 minutes.
Remove the soufflé from the ramekin, garnish to taste, and serve right away.
Notes / Tips / Wine Advice:
Since the chocolate soufflé batter can be prepared days early and frozen, it is perfect for when you are expecting guests. In this case, you simply have to bake for 5 minutes longer.