Separate your eggs and set aside. Melt your butter in the microwave oven (make sure to use a microwave-safe cup).
Place your chocolate pieces and a tablespoon of water in a microwave-safe bowl and melt for 2 minutes, mix chocolate and melt again for 1 minute. Mix so that it makes a nice paste. Mix in butter and set aside.
Using an electric mixer, whip the egg whites with a pinch of salt until they are really stiff. Beat your egg yolks, adding in sugar and your chocolate mixture. Mix well.
Gently pour your chocolate batter into the stiff whites. Once the egg whites are completely incorporated, pour into a serving bowl and cover with aluminum foil. Leave in the refrigerator for at least 3 hours.