The key to a successful chocolate mousse is making sure the egg yolks and whites are completely separate. Mix the yolks with the melted chocolate, and only add the whites in once they have been beaten. Tested and approved!
Prepare a bain-marie (double boiler). Place a mixing bowl over a pan half filled with water.
Melt the chocolate pieces and the diced butter. Then add the sugar and stir until you have a smooth paste.
Remove from heat and allow the mixture to cool slightly.
Meanwhile, separate the egg whites from the yolks.
Add the yolks to the chocolate mixture and stir with a wooden spoon. Beat the egg whites until stiff with an electric mixer.
Gently fold the egg whites into the chocolate mixture in 2 or 3 stages.
Place the mousse into ramekin dishes and chill for at least 2 hours before serving.