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 sugar, sprinkling 1/4 at a time and beating well after each addition, until stiff peaks form. With a large silicone spatula, gently fold in vanilla. Gently fold in flour mixture 1/4 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.