1. Blend the egg white and 15 ml/1 tbls of the water in a basin until smooth. Stir in the remaining water and the sugar.
2. Place the basin over a saucepan of boiling water and whisk with an electric mixer for about 5 minutes or until the mixture thickens.
3. Remove the bowl from the heat and whisk for a few minutes more, then spread quickly over the cake and leave to set.