Step 1Preheat oven to 350°F. Prepare an ungreased 10-inch tube pan.Step 2In a small bowl, stir together powdered' sugar and flour; set aside. Step 3In a large bowl, add egg whites, cream of tartar and vanilla. With mixer at high speed, beat until soft peaks form. Gradually add granulated sugar, about 2 tablespoons at a time, beating until stiff peaks form.Step 4With rubber spatula, fold in flour mixture, about 1/4 cup at a time, just until flour disappears. Pour mixture into prepared pan, and with spatula, cut through batter to break any large air bubbles.Step 5Bake 40 to 45 minutes, or until top of cake springs back when lightly touched with finger.Step 6Invert cake in pan on funnel; cool completely. Using a narrow spatula, loosen sides of cake from pan; remove cake to a serving platter.