Melt the chocolate and the butter bain-marie (hot water bath).
In the meantime, beat the 6 egg whites. When it starts foaming, add the sugar gradually. (I use my Kitchenaid so I have my hands free to deal with the chocolate).
Once the chocolate/butter mixture is melted, remove from the heat and add the egg yolks (3). Mix well.
Once your egg whites are stiff and firm, incorporate the chocolate by folding carefully with a spatula.
Pour in ramekins or any other dish of your choice and reserve in the fridge for an hour.