Heat oven to 350 degrees. Sift flour into a mixing bowl, then resift flour with 3/4 cup sugar.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg whites on medium speed until frothy. Add cream of tartar; beat until whites have tripled in size, about 3 minutes. Slowly add remaining 3/4 cup sugar; beat until firm and glossy, 1 to 2 minutes.
Transfer egg whites to a large metal mixing bowl. Gently fold in flour mixture in three additions, mixing from bottom of bowl (try to use fewest strokes possible). Fold in grated chocolate, and mix. Pour batter into a 10-inch nonstick angel-food-cake pan, and run a knife through center of the batter to release any air bubbles. Bake until top becomes golden brown, 40 to 45 minutes.
Invert the pan over the neck of a bottle, and cool about 1 hour. Loosen the cake from the pan with a flexible knife, and turn the cake over onto a cake stand.
Make ganache: Heat buttermilk over medium heat; add chopped chocolate, and stir slowly until melted. Since buttermilk will curdle slightly, strain mixture through a very fine sieve, and stir slowly until smooth. Spread ganache over top of cooled angel-food cake, and allow to harden slightly before serving.