Cook first 3 ingredients in a heavy saucepan over medium heat, stirring constantly, until sugar dissolves. Bring to a boil, and boil until a candy thermometer registers 248° (about 5 minutes).
Beat egg whites at high speed with an electric mixer until foamy; add salt, and beat until stiff peaks form. Gradually whisk hot syrup into egg whites, whisking until stiff peaks form and frosting is of spreading consistency.
Stir in vanilla and, if desired, food coloring.
