Dissolve the sugar in the water over low heat.
Stir in a pinch of cream of tartar and bring to a boil.
Boil rapidly until the syrup reaches 240°F (115°C) on a candy thermometer.
Meanwhile, beat the egg whites in a bowl until stiff.
Beating constantly, pour the syrup onto the egg whites.
Continue beating until the icing is thick enough to spread.
Beat in the flavoring.
To use, spread on the cake with a spatula dipped in hot water.
Swirl icing to imitate a snowdrift or spread smoothly.