Soak the gelatine sheet in cold water.
Dissolve the icing sugar in the water and bring to the boil. Remove from the hob, squeeze the water the gelatine sheet and add it to the pan while stirring.
Fold in the sparkling wine and pour into four small tins.
Cover with aluminium foil and put down the spoon. Place in the freezer.
Bring the frozen raspberries to the boil and leave to cool.
Mix the raspberries with the icing sugar and press through a fine-mesh sieve.
Serve the champagne sorbet with the raspberry sauce and chopped white chocolate.