In a heavy saucepan combine sugar, water and cream of tartar. Slowly bring mixture to a boil, stirring sides of the saucepan with a brush dipped in water to remove any sugar crystals. Cover and boil for about 3 minutes or until a candy thermometer registers 245 degrees.
Beat egg whites with pinch of salt until glossy and peaked. Slowly pour in syrup as you continue to beat the whites. Beat until cool and frosting is soft and fluffy. Fold in vanilla.