Using an electric mixer beat the egg whites in a bowl until they start to foam, then add 450g/1lb icing sugar and beat on a high speed until the mixture starts to look thick and glossy.
Stir in the remaining 100g/3½oz icing sugar and the glycerine until well combined then beat again until the mixture forms soft peaks when the whisk is removed. (For a thicker icing, add more icing sugar.)
The icing will keep well for 1-2 days if the bowl is scraped down inside the bowl and covered with a damp tea-towel. Store in a cool part of the kitchen.