In top of a double boiler, combine chocolate pieces, sugar and butter. Melt over barely simmering water, stirring constantly. Remove from heat while a couple of chunks are still visible. Cool a little bit, stirring occasionally. Separate whites and yolks. Lightly whisk yolks with an orange juice and zest. Mix melted chocolate and yolk mixture. In a medium bowl beat egg whites until soft peaks. Carefully mix chocolate mousse and whipped whites. Transfer the mousse into a serving bowls, topped with a whipped cream and refrigerate for at least 2 hours, .