Combine sugar, water, and salt in a medium-size heavy saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and sugar is dissolved. Continue cooking, stirring frequently, until mixture reaches soft ball stage (240°). Stir in bourbon and flavoring.
Beat egg whites (at room temperature) until foamy; add cream of tartar. Beat at medium speed of electric mixer, slowly pouring hot syrup in a thin stream over egg whites. Turn mixer to high speed, and continue beating until stiff peaks form and frosting is thick. Spread immediately on cooled cake.