Break the chocolate into a basin and set over a saucepan half filled with hot water. Stir occasionally until the chocolate has melted. Stir in the butter and blend well. Separate the eggs, adding the yolks to the chocolate mixture and placing the whites together in a basin. Stir to blend the egg yolks and chocolate and remove the basin from the heat.
Whisk the egg whites until stiff and, using a metal spoon, fold gently but firmly into the chocolate mixture. Pour the mousse into individual serving dishes and chill for several hours. It is always advisable to make this kind of mousse several hours in advance; thorough chilling is required to develop a firm texture. For a party, the dishes look pretty if decorated with a little whipped cream or small pineapple cubes. The sharp taste of the pineapple makes a pleasant contrast to the rich chocolate flavour.