Break up 225 g/8 oz/2 cups of the white
chocolate and place in a bowl with the butter. Place over a pan of hot
water or in the microwave to melt. Beat in the egg yolks. Whisk the
egg whites until stiff and fold into the chocolate mixture. Turn into
six ramekins (custard cups) and chill until set. Coarsely grate the
remaining white chocolate and use to decorate the mousses with the
grated plain chocolate.