1- Separate the egg yolks from white. In a large salad bowl, whisk together the yolks and the sugar until the mixture gets clearer and smooth.
2- Cut the chocolate and butter in small squares and put them in a small heavy saucepan with 2 spoons of Cointreau and 2 spoons of water. Melt the whole over a low heat, stirring non stop with a wooden spoon until smooth. The chocolate must not stick to the pan.
3- Pour the melted chocolate in the egg and sugar mixture and stir ; add the left spoon of liquor, mix, add the cream and stir.
4- Beat the egg whites with an electric mixer until frothy. They must get "hard" (the real test is to turn the bowl upside down and the whites should not fall down !!!).
5- Spoon by spoon (quarter by quarter) , gently incorporate the egg whites to the chocolate mixture; you can do it simply with a whisk (you just need to do quick and light moves, you must not "break" the bubbles kept in the whites).
6- Evenly pour the mousse in the 4 soufflé dishes (you can as well use 6 to 8 ramequins to serve more guests). Leave in fridge for at least 3 hours before serving (the mousse should get "hard").
Decorate with orange strips, a teaspoon of sour cream or coffee beans...