Melt the chocolate bits in a heavy-bottomed pan over very low heat, stirring often to prevent burning; set aside. Beat the egg yolks until pale and lemon-colored. Slowly stir in the chocolate and blend well. Beat the egg whites until stiff but not dry. Add a third of the whites to the yolks and chocolate, add the vanilla, mix well, then carefully fold in the remaining whites. Spoon into individual serving dishes or a serving bowl. Cover and chill at least 8 hours before serving.