Position a rack in the lower third of the oven and preheat the oven to 375 degrees . Pulse the chocolate in a food processor until finely ground; reserve. In a medium bowl, whisk together the flour and 3/4 cup of the sugar until combined; reserve.
In the bowl of a mixer fitted with a whisk, mix the egg whites, cream of tartar and salt on low speed until frothy, about 1 minute. Increase the speed to medium and beat the egg whites until they hold soft peaks, about 2 minutes. Stir in the remaining 3/4 cup of sugar, then beat the egg whites at medium-high speed until fluffy and just stiff.
Remove the bowl from the mixer, sift one-third of the flour mixture over the egg whites and fold it in with a large spatula until just blended. Add the vanilla and fold in the remaining flour in 2 batches. Fold in the reserved ground chocolate and scrape the batter into a 10-inch tube pan. Bake until a toothpick inserted in the center comes out clean, about 45 minutes. Invert the pan and place the tube opening over the neck of a bottle to suspend the cake, then let cool completely, at least 1 hour and up to overnight.
Unmold the Chocolate Chip Angel Food Cake and coat generously with the Seven-Minute Frosting.
The Do's and Don'ts of Cooking with Chocolate