Melt the chocolate and butter over a pan of simmering water in a heatproof bowl.
When its melted, take it off the heat and leave it to cool.
Add the vanilla extract to the chocolate mixture.
Separate the eggs, and whisk the sugar and yolks together. Then, whisk the whites until they form stiff peaks and there is no liquid left at the bottom of the bowl.
Whisk the yolks into the chocolate mix.
Add a tablespoon of whisked white to the bowl and whisk it in. Then, add the rest in one go and fold it in.
Put it in sundae glasses or ramekins or one big serving bowl. I whipped some cream and squeezed it through a piping bag on top of the mousse, or you could use canned squirty cream to decorate.
Put it in the fridge to chill and set for a few hours.