Melt the 400gr of black chocolate in a pot with a splash of water over low heat and then let it cool.
In a bowl, separate the egg whites from the egg yolks. Incorporate the egg yolks to the melted chocolate.
Beat the egg whites until stiff and progressively add them to the mixture (gently circle the stiff peaks with the mixture).
Put the mousse in the fridge over night. Take it out of the fridge 30min before serving.