In a bowl, add dark chocolate and butter in small pieces, and melt in a bain-marie or in the microwave for 1 minute.
Separate the egg whites from the yolks
In a separate bowl, beat the egg whites until stiff with a hand held mixer. Add in one teaspoon of sugar at the end until dissolved.
Add the egg yolks to the chocolate one by one.
Add 1/3 of the meringue and stir, with the wooden spoon, quickly and in circles until integrated. Once integrated, add in another third of meringue and stir more slowly until combined. Then, carefully add the remaining meringue and combine both mixtures with slow folding movements until well integrated.
Pour the mixture into small glasses, cover them with cling film and put them to chill in the freezer for 1 hour.
Take them out of the freezer and sprinkle chocolate on top to decorate and give a crunchy touch to the dessert. Et voilà! You have made a delicious chocolate mousse.