Place the egg
white with the water, golden syrup and cream of
tartar in a heatproof bowl. Whisk together well
until thoroughly blended.
Stir the
icing sugar into the mixture and place the bowl
over a saucepan of simmering water. Whisk until
the mixture becomes thick and white.
Remove the bowl
from the saucepan and continue to whisk the frosting
until cool and thick, and the mixture stands up in
soft peaks. Use immediately to fill or cover cakes.