Chocolate Angel Food Cake

I've been wanting to make this cake ever since I got a killer deal on a ton of strawberries at grocery outlet. This was the first time I have ever used 12 eggs at one time and the first time I have ever beat egg whites to form stiff peaks.
I got the recipe here. 
This is probably the most difficult cake I have ever made, not that it was that difficult but I have just never made anything like it before.

Ethereal Angel Food Cake

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

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

Preheat the oven to 325 degrees.
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.
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.

The flour, sugar, cocoa mixture

All the egg shells, except a few...

The egg whites and vanilla

My stiff peaks

Folding in the dry ingredients

Putting it in the bunt

In the end the cake was delicious, but was a half fail. I think the oven and the cake joined forces to trick me. (my oven is old and sad. Since we moved I have to decrease the temp 25 degrees and decrease the time a little to avoid burning. I miss my brand new oven in the old apt.) According to the time, the color of the cake, the springy-ness of the cake, and the crispness in some areas, the cake was done. Sadly two minutes out of the oven it's size was reduced by half. I later discovered the very bottom of the cake was not cooked completely. Well we were going to eat it either way, so I cut off the bad and ate the good, and it was very tasty. I'm not too disappointed because I had already prepared myself for a flop, considering I had never made a cake like this before and I had to use a bunt instead of a special angel food cake pan like the recipe wanted.

Oh well.