1. In a very lg mixing bowl allow egg whites to stand at room temp. for 30 mins. Meanwhile, sift pwdr'd sugar and flour together three times; set aside.
2. Add cream of tartar and vanilla to egg whites. Beat with an electric mixer on med. speed until soft peaks form (tips curl). Gradually add granulated sugar, about 2 tbsp at a time, beating until stiff peaks form (tips stand straight).
3. Sift one-forth of the pwdr'd sugar mixture over beaten egg whites; fold in gently. Repeat, folding in the remaining pwdr'd sugar mixture by fourths. Pour into an ungreased 10-in tube pan. Gently cut through batter to remove air pockets.
4. Bake on the lowest rack in a 350 F oven for 40 to 45 mins or until top springs back when lightly touched. Immediately invert cake; cool thoroughly in pan. Loosen cake from pan; remove cake.