Combine sugar and water in a saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and sugar is dissolved. Continue cooking, stirring frequently, until mixture reaches thread stage (234°).
Beat egg whites (at room temperature) until stiff but not dry. While beating at medium speed of electric mixer, slowly pour hot syrup in a thin stream over egg whites. Turn mixer to high speed, and continue beating until mixture is cooled, about 5 minutes.
Combine egg yolks, vanilla, and salt; beat until thick. Add to egg white mixture; beat well. Sprinkle flour over egg white mixture, 1/2 cup at a time, folding well after each addition.
Pour batter into an ungreased 10-inch tube pan, spreading evenly with spatula. Bake at 325° for 50 minutes or until cake springs back when lightly touched. Remove from oven, and invert pan. Cool 40 minutes; remove cake from pan.
Note: Delicate Sponge Cake may also be baked in two 9-inch round cake pans.