In large bowl, beat egg whites, cream of tartar and salt with electric mixer on low speed until foamy. Increase speed to medium and beat until thick and frothy. Increase speed to high and gradually beat in remaining 3/4 cup sugar, beating until stiff peaks form. Gently fold in lemon and vanilla. Gently fold flour mixture, 1/4 at a time, into egg whites. Spoon batter into ungreased pan.