In large bowl, beat egg whites and salt with electric mixer until foamy; add cream of tartar and beat until soft peaks form. Gradually beat in 1 cup granulated sugar, adding 1/4 cup at a time, beating well after each addition until soft peaks form. With large spatula, gently fold in vanilla extract. Gently fold in flour mixture, 1/4 cup at a time, over egg whites with an over and under motion after each addition. Fold additional 10 strokes after last addition until flour is well blended. Gently transfer batter to ungreased pan. With spatula or knife, cut through batter gently.