Chocolate Angel Food Cake

1 cup cake flour
1/4 cup unsweetened cocoa powder
1/3 cup granulated sugar
11 large egg whites at room temperature
1 teaspoon cream of tartar
3/4 cup granulated sugar
1 teaspoon vanilla extract
1/2 teaspoon chocolate flavor optional

Vanilla Glaze
1/2 cup powdered sugar
1/4 teaspoon vanilla extract
2 teaspoons water up to 3 tsp.
Stir until smooth and pour over cake.

Preheat oven to 350ºF. 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. 

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.