In advance, measure egg whites and allow to come to room temperature.
Place oven rack in middle position and preheat to 375 °F. Grease bottom only of 10-inch tube pan.
In small bowl, add confectioners' sugar and cake flour; stir to combine and set aside.
In the large bowl of a stand mixer, add egg whites, cream of tartar, and vanilla extract. At high speed, beat until well mixed.
With mixer on high speed, add granulated sugar, 2 tablespoons at a time. Beat until granulated sugar is just dissolved (test by rubbing a small amount between fingers) and whites form stiff peaks but are not dry. Do not scrape bowl during beating.
Using large whisk, gently fold in flour-sugar mixture, about 1/4 at a time, just until flour disappears.
Pour mixture into prepared 10-inch tube pan. Distribute evenly, then using a long narrow metal spatula or large knife, cut through batter to eliminate any large pockets of air.
Bake 35 minutes or until cake springs back when lightly touched with finger. Any cracks on surface should look dry.
Invert cake in pan, using metal funnel, if necessary. Cool completely. Use metal spatula to loosen cake from pan and invert onto serving plate. If desired, sprinkle with confectioner's sugar or frost with whipped cream. Serve with whipped cream or ice cream and sliced fresh fruit or berries.
-