Place the egg whites in a bowl, then stir in the icing sugar, a spoonful at a time, until the icing falls thickly from the spoon.At this stage, stop adding any more sugar and whisk with an electric whisk for 10 minutes or until the icing stands up in stiff peaks, then stir in the glycerine. Now spread the icing all over the top and sides of the cake as evenly as possible using a palette knife. Switch to a broad-bladed knife (or smaller palette knife) to 'spike' the icing all over to give a snow-scene effect. Leave the cake overnight for the icing to dry out before placing it in a container till needed.Note: this recipe contains raw egg.