Separate the eggs (yolks in one bowl, whites in another)
Whip the yellow yolks with the sugar until very light and fluffy.
Melt chocolate with the butter in a bain marie or on very low heat. Cool to room temperature.
Mix the yolk/sugar with the cooled chocolate/butter mixture.
Whip egg whites until soft peaks form.
Fold the chocolate/yolk mixutre into the whipped egg whites gently until uniform in colour.
Refridgerate until set.