Chocolate Angel Food Cake

Note: To make a chocolate version, substitute 1/4 cup cocoa powder for 1/4 cup of the flour.

Ingredients

Dry ingredients:
• 1 cup all-purpose flour
• ¾ cup granulated sugar
• ½ teaspoon salt
For the egg white mixture:
• ¾ cup granulated sugar
• 12 large egg whites (make sure not to get any of the shell or egg yolk in with the whites or they won’t beat up properly)
• 1 teaspoon vanilla extract
• 1 ½ teaspoon cream of tartar

Directions

1. Preheat the oven to 325 degrees.
2. In a medium bowl, whisk together the flour, cocoa powder (if using), sugar and salt and set aside. In another bowl place the egg whites and add the vanilla. With a hand mixer (or with a stand mixer), beat the egg whites and vanilla on medium-high until the mixture is just frothy, about one minute. Sprinkle the cream of tartar on the top of the foamy whites and continue beating on medium-high until soft peaks form, another 2-3 minutes.
3. Add the sugar 1/4 cup at a time until fully incorporated. Continue beating until the whites are stiff and glossy (see the picture below). This may take several minutes, depending on the type of mixer you are using. With a whisk, gently fold the dry ingredients into the beaten egg whites. Pour the batter evenly into an ungreased angel food cake pan and smooth the top with a rubber spatula. Place the cake on a rack in the center of the oven and bake for 40-45 minutes, until the top of the cake is golden brown, the cake springs back when lightly touched and the cracks are dry to the touch. Place the cake upside down on cooking rack or bottle until cool (see picture below). Slide a knife around the edges of the pan and gently remove the cake.

from: 
http://www.melskitchencafe.com/2010/06/ethereal-chocolate-angel-food-cake.html