Beat egg whites until foamy. Add cream of tartar and salt; beat until soft peaks form. Add sugar, 2 tablespoons at a time, beating until stiff peaks form. Sprinkle flour over egg white mixture, 1/4 cup at a time; fold in carefully. Fold in vanilla. Pour batter into an ungreased 10-inch tube pan, spreading evenly. Bake at 375 degrees for 30 to 35 minutes or until cake springs back when lightly touched. Invert pan; cool 40 minutes. Loosen cake from sides of pan 10-inch cake.