Crumble the chocolate and melt it in a water bath with 1 tbsp water.
Remove from the stove, add the butter and stir with a wooden spoon. Add the yolks while stirring nonstop.
Beat the whites to stiff snow with the powdered sugar and mix carefully with the chocolate sauce, stirring from the bottom going up.
Distribute into cups and leave in the fridge for 2 hours.