Dark Chocolate Angel Food Cake

3/4 cup all-purpose flour
4 tablespoon cocoa
1 1/4 cups egg whites (10 large or 13 small eggs)
1/4 teaspoon salt
1/4 teaspoon baking soda
1/2 teaspoon baking powder
1 1/4 cups granulated sugar, sifted
1 teaspoon vanilla

Sift the flour once; measure it; add the cocoa and sift three more times. Beat the egg whites and the salt until they are foamy, add the soda and baking powder and beat until the eggs will just hold a peak, but are not dry. Fold in the sugar carefully, two tablespoons at a time. Fold in the vanilla. Sift a small amount of flour over the mixture and fold it in carefully; continue to add flour gradually until all the flour is incorporated. Pour the batter into an ungreased 10-inch tube pan. Bake for one hour, the first 30 minutes at 275 degrees , the next 15 minutes at 300 degrees and the last 15 minutes at 325 degrees. Remove the pan from the oven and invert it until the cake is cool. Then remove the cake from the pan.