Sift together the flour, cocoa, and first amount of sugar 3 times; set aside.
In a large mixer bowl, beat egg whites and salt until foamy on medium speed. Sprinkle in the cream of tartar and the vanilla. Continue to beat until egg whites are stiff and stand in peaks; about 2-3 minutes.
Add the 2nd amount of sugar; 2 Tablespoons at a time. Beating only until sugar is blended, about a minute.
Sprinkle flour mixture, ¼ cup at a time over the beaten egg whites, folding it in gently with a spatula, just until flour mixture disappears.
Gently add the batter into an ungreased 10 inch tube pan. With a knife, cut through the batter 2 or 3 times in a circular motion.
Bake 30-35 minutes. Turn oven off and leave in oven for 5 minutes more.
Remove and invert tube pan on a long neck glass bottle like a coke or beer bottle. Let cake hang until it is completely cooled. When cooled, carefully take a sharp knife and loosen the cake from around the pan to remove it. Place angel food topside down on a plate or a cake plate.
If you do not like Ganache, you can also serve the cake plain, or topped with whipped cream or your favorite glaze or icing.
Place chocolate in a stainless steel bowl.
Heat the cream, butter, and sugar in a 2-½ quart pan over medium heat. When mixture is hot, stir to dissolve the sugar and bring to a boil.
Pour boiling cream over chocolate and allow to stand for 5 minutes without stirring.
After the 5 minutes, stir until smooth. Allow chocolate to cool at room temperature.
Drizzle the ganache over cake, or ice the cake completely with the chocolate ganache.
You can also chill some of the ganache for about 10 minutes in a pastry bag for piping decorations on the cake.