Makes 600ml (1 pint)
This delicious soft icing is a little tricky to make, so follow the instructions exactly. Quick and accurate decisions are necessary in judging when the icing is ready and then it must be applied to the cake immediately. If the icing is not cooked enough, it will still taste good, but will not dry out properly on the outside. If cooked too much, it will be difficult to spread over the cake.
Ensure your cake is ready before you start, as this icing begins to set very quickly. Bring to the boil a saucepan of water large enough to hold a heatproof bowl. Place the egg whites in the bowl and whisk with a hand-held electric beater until very stiff.
In a separate saucepan over a medium-high heat, dissolve the sugar in the water and boil for 5-10 minutes until the liquid is thick and syrupy and has reached the 'thread' stage - when the last few drops that fall from a metal spoon dipped into the syrup come off in one long, quite thick and syrupy thread.
Pour the boiling syrup over the stiffly beaten egg whites, whisking all the time with the hand-held beater. Place the bowl in the saucepan of simmering water. Continue to whisk over the water for 10-15 minutes until the icing is snow white, very thick and meringue-like.
Spread quickly over the cake with a palette knife, regularly dipping the knife into a jug of boiling water. The icing sets very quickly at this stage, so speed is essential.