This quantity is sufficient for a 9-inch two-layer cake or the equivalent.

Steps

1. In a saucepan, combine the sugar, water, and cream of tartar. Bring the mixture slowly to a boil, stirring once or twice to dissolve the sugar before it bubbles. Cover, and boil 3 minutes, then uncover and boil to 245 degrees.
2. Remove the pan from the heat and pour the syrup in a thin stream over the stiffly beaten whites in a mixing bowl. Add the salt and vanilla and continue beating until the frosting cools and is firm enough to spread. If the frosting stiffens excessively, beat in about ¼ teaspoon boiling hot water.