Preheat oven to 350F. Lightly grease a 10 cup pan with a removable bottom and dust with cocoa powder. Resift the cake flour five times with the cocoa powder and 1/3 cup granulated sugar. Using 1/4 of the egg whites at a time, whip all the whites until foamy. (See below for egg white notes.) Add the cream of tartar when all 11 egg whites are foamy. Continue to whip the egg whites, adding 1/4 cup sugar, 1 tablespoon at a time. Stir the vanilla extract and chocolate flavor into the beaten egg whites. Sift 1/4 of the flour mixture into the batter and fold gently. Repeat until finished with the flour. Pour the batter into the prepared tube pan. Gently move a knife through the batter to remove air pockets. Bake in the preheated oven for 45 minutes. Invert pan and let cool for 1 1/2 hours. Remove from pan and drizzle with vanilla glaze.
Beating Egg Whites: Use a deep glass bowl and a thin wire whisk. Do not use an aluminum or plastic bowl. Since this recipe calls for cream of tartar, do not use a copper bowl. The egg whites whould be 75F in temperature and should contain no egg yolk.
For glaze, stir all ings. until smooth and pour over cake.