Using the whisk attachment, beat the whites until frothy. Add cream of tartar and continue beating on medium high speed, gradually adding the remaining 1/2 cup sugar. Keep beating until the whites form glossy peaks that are stiff, but not dry. Gently scrape the beaten whites into another bowl and set aside.
Combine the yolks, sifted dry ingredients, 1/4 cup hot water and vanilla in the mixing bowl. Using the paddle attachment, beat at medium high speed for 4 minutes, or until light-colored and fluffy. When you lift the beater, the batter should slowly fall back into the bowl like ribbon folding upon itself.
Add about one-third of the beaten whites to the yolk mixture and gently combine to lighten the mixture. Fold in the remaining whites, cutting down through the mixture with the spatula, moving across the bottom and coming back up to fold the mixture over the top. Repeat, turning the bowl, until the ingredients are evenly distributed with no visible streaks. The goal is to combine the mixtures without deflating.
Gently scrape the batter into an ungreased, 10-inch tube pan with a removable bottom. Using a dinner knife, slowly crisscross through the batter several times to eliminate any large bubbles.
Bake for 45 to 50 minutes, not opening the oven door until at least 40 minutes have passed. The cake is done when the top no longer appears moist in any cracks.
Invert cake pan over a bottle and let it rest upside-down until thoroughly cooled, about 2 hours.