Preheat the oven to 350ºF. Set aside a 9-inch angel food or tube pan. Do not grease the pan.
In a small bowl, sift together the flour, cocoa powder, 1/2 cup sugar, and salt. Set aside.
Pour the egg whites into the bowl of a stand mixer. Whip the egg whites on medium speed. When they look foamy add the cream of tartar.
Increase the mixer speed and continue to whip the egg whites until they hold soft peaks. Continue whipping as you gradually add the remaining 1 cup of sugar. Make sure to not overwhip; the eggs should be soft and not stiff. Mix in the vanilla.
Use a spatula to fold the dry ingredients into the egg whites, folding from the sides and bottom to mix while not deflating the whites.
Pour the batter in the pan and smooth the top. Bake for 45 minutes.
Remove from the oven and immediately invert over a cooling rack with the center hole of the cake pan resting on the neck of a heavy bottle or a metal funnel. Let the cake cool completely before removing from the pan and serving.