Melt the chocolate in a bain-marie. We have a bain-marie pan, but you can use 2 pans : place the little one in the big (filled half with water).
Add the butter (cut in little pieces) and mix always in the bain-marie. Melt it good.
Separate the eggs white from the yolk. Beat the eggs white with a pinch of salt.
Place the chocolate mixture in a terrine and let cool. Add the eggs yolk.
Add the eggs white with the chocolate mixture
Put the mousse in ramekins and let it cool in the fridge 4 hours.