Preheat oven to 350°.
Sift powdered sugar & flour together 3 times over a sheet of waxed paper. Beat egg whites, cream of tartar, & vanilla in large, cold bowl on med speed until soft peaks form (tips curl).
Gradually add sugar, beating until stiff peaks form (tips stand straight).
Sift about 1/4 flour/sugar mixture over beaten egg whites; fold-in gently. Repeat, folding-in remaining flour mixture 1/4 at a time. Transfer batter to ungreased 10” tube pan.
Bake on lowest rack in oven 40-45 min (until top springs back when lightly touched). Immediately invert cake (leave in pan); cool thoroughly. Loosen sides of cake from pan; remove cake.