Beat egg whites, cream of tarter, salt, and vanilla with wire whisk in large bowl to soft peaks. Gradually add sugar, a little at a time and beat until stiff. Sift together powdered sugar, and cake flour and add gradually folding into mixture. Pour into ungreased tube pan. Bake on lowest oven rack at 350°F for about 40 minutes. Invert and let cool before loosening from pan.