Preparation
Take the butter and eggs out of the refrigerator at least 1 hour before preparing the chocolate mousse.
Prepare a bain-marie: place the container in a saucepan place half-filled with water placed on a heat source. Add the butter and the chocolate pieces in this container and melt it while stirring with a wooden spoon. Add the sugar and stir until you have a smooth chocolate cream.
Remove the container from the water bath and let the mixture cool slightly. Add the egg yolks and stir with the wooden spoon.
Put the egg whites in another container and beat them until stiff. Incorporate the whisked whites into the preparation in 2 or 3 batches, lifting it and above all without turning, because the snow on the eggs must not be broken.
Once finished, put the mousse in the refrigerator for at least 2 hours.
The ideal mousse is made the day before.