Preheat & Prepare Pan: Preheat oven to 325°F (165°C). Use an ungreased 9 or 10-inch tube pan (Angel food cake pan). Do NOT grease the pan.

Whip Egg Whites: In a very clean, dry, large mixing bowl (preferably glass or metal), beat the egg whites and cream of tartar (and optional salt) with an electric mixer on medium speed until foamy.

Add Sugar Gradually: Increase mixer speed to medium-high and gradually add the granulated sugar, about 1-2 tablespoons at a time, beating well after each addition. Continue beating until stiff, glossy peaks form (when you lift the beaters, the peak should hold its shape and maybe curl slightly at the tip).

Add Vanilla: Gently beat in the vanilla extract.

Fold in Flour: Sift the cake flour over the egg white mixture in 2-3 additions. Gently fold the flour into the egg whites using a large rubber spatula with a light touch. Use an up-and-over motion, rotating the bowl, until just combined and no flour streaks remain. Do not overmix, or you will deflate the egg whites. (If making chocolate version with cocoa, sift it with the flour. If using melted chocolate, fold it in very gently after the flour).

Fill Pan: Gently spoon or pour the batter into the ungreased tube pan, spreading it evenly.

Bake: Bake for 40-45 minutes, or until the top is golden brown and springs back when lightly touched. A wooden skewer inserted into the center should come out clean. Avoid opening the oven door during the first 30 minutes of baking.

Cool Upside Down: Immediately upon removing from the oven, invert the tube pan (hang it on the neck of a bottle or use the pan's feet if it has them) and let the cake cool completely upside down in the pan. This prevents it from collapsing. This can take 1.5 - 2 hours.

Release Cake: Once completely cool, carefully run a thin knife or offset spatula around the edges of the pan (both center tube and outer edge) to loosen the cake. Invert onto a serving plate.

Serve: Serve plain, or dust with powdered sugar, or top with fresh fruit and whipped cream.