Everyone's favourite baker Mary Berry gives you the lowdown on making the perfect royal icing.
Sieve the icing sugar.
Whisk the egg whites in a large bowl until they become frothy.
Add the icing sugar to the egg whites, a spoonful at a time, and fold in.
Add the lemon juice and glycerine and stir.
Beat the icing until it is very stiff and white and stands up in peaks.
Spread over the top and sides of the cake and rough up the icing with a spatula so that it forms peaks. (If you want a smooth icing you may need to thin the icing down a little with more egg white and use to ice the sides and top of the cake using a palette knife.)
Transfer to a cake tin and seal up until ready for use.
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James is joined by chefs Gennaro Contaldo, Antonio Carluccio and Giancarlo Caldesi.