Prepare a saucepan of water and a heatproof bowl for a double boiler. Bring the water to a boil. Meanwhile, roughly chop the chocolate.

Add the butter to the bowl first, then the chopped chocolate. Place the bowl over the simmering water. Let it melt gently, stirring occasionally. Remove from heat once the chocolate is melted and let it cool slightly.

Separate the egg yolks from the whites. In a separate bowl, combine the yolks and sugar (add the vanilla sugar if using). Whisk until the mixture is pale and creamy.

Gently fold the beaten egg whites into the chocolate mixture in three parts. Mix carefully, then spoon the mousse into serving glasses or bowls. Chill in the refrigerator for at least 5 hours, or overnight for the best texture.