Combine sugar and water in a 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 thread stage (234°).
Beat egg whites (at room temperature) until stiff but not dry. While beating at medium speed of electric mixer, pour hot syrup in a thin stream over egg whites. Turn mixer to high, and beat 5 minutes or until cool. Stir in flavoring.
Beat egg yolks until thick and lemon colored; add to whites, and beat well. Sprinkle flour over egg white mixture, 1/2 cup at a time; fold well after each addition.
Pour batter into an ungreased 10-inch tube pan, spreading evenly. Bake at 325° for 50 minutes or until cake springs back when lightly touched. Remove from oven: invert pan. Cool 40 minutes; remove from pan.