Place fresh cold egg whites in the bowl of a heavy-duty mixer. Set aside until they reach 60 degrees, slightly below room temperature.
Pour flour, powdered sugar, salt and cream of tartar into a triple sifter. Sift onto a sheet of waxed paper; set aside.
Adjust rack in lower third of oven. Preheat oven to 350 degrees. Beat egg whites, preferably with a whisk attachment, until frothy. Add cream of tartar and whisk on medium speed until soft peaks form. Gradually add granulated sugar, whisking until the whites form soft, droopy white peaks. Add vanilla and lemon zest in the final moments of beating.
Sprinkle a quarter of the flour mixture over the whites. Using a rubber spatula, fold it into the whites. Repeat with the remaining flour mixture, folding in a quarter of the mixture at a time.
Gently pour the batter into an ungreased 10-inch tube pan.
Bake 40 to 45 minutes, or until the top is light golden, the cake feels spongy and springs back when touched, and a wooden toothpick inserted in the middle comes out free of cake. Invert the cake (in its pan) onto the neck of a bottle or funnel. Let cool completely before removing from pan.
To remove, slip the tip of a small metal spatula between the cake and the pan. Slowly work tip around the perimeter to release any cake sticking to the top of the pan. Tilt pan on its side and gently tap it against a counter to loosen the cake. Rotate pan, tapping as you turn, until the cake appears free. Cover with a rack, invert and tap pan firmly to release the cake.