How to Make It
Preheat the oven to 350°. Sift 2/3 cup of the superfine sugar onto a piece of wax paper or parchment; set aside. Sift the remaining 1/3 cup superfine sugar with the cocoa powder, cake flour and salt; set aside.
In the large bowl of a standing electric mixer, beat the egg whites at medium speed until foamy. Add the cream of tartar and beat until the foam is white and holds the lines from the beater. Increase the speed to medium-high and add the sifted superfine sugar, 1 tablespoon at a time, until completely incorporated. Beat for 30 seconds longer until soft peaks form.
Remove the bowl from the mixer. Sift and fold the dry ingredients into the beaten whites, folding them in with a large rubber spatula. (It may be easier to have someone else sift while you fold.) Pour the batter into a 10-cup angel food cake pan or bundt pan and gently smooth the top without deflating the mixture. Bang the pan once (not too hard) on your work surface to release any large air pockets.
Bake the cake in the bottom third of the oven for about 35 minutes, or until a tester inserted in the center comes out clean. Invert the cake pan onto its feet, if it has them, or over a narrow-necked bottle. Let the cake cool completely in the pan, about 1 hour.
Run a thin flexible metal spatula or knife between the cake and the sides and center tube of the pan to loosen (you may need to jiggle the pan to completely release the cake). Invert the cake onto a cardboard cake circle or serving platter.
Put the egg whites in the large bowl of a standing electric mixer. In a small heavy saucepan, stir together the granulated sugar and 1/3 cup of water. Bring to a boil over moderately high heat, washing down the saucepan with a wet pastry brush.
When the sugar syrup comes to a boil, begin beating the egg whites at medium high speed. When they become foamy, add the cream of tartar and beat at medium speed until stiff peaks form.
Insert a candy thermometer into the sugar syrup (be sure the bulb is completely submerged). Cook until the syrup reaches 242°. Remove from the heat. With the mixer still on, beat the sugar syrup into the whites in a slow stream until completely incorporated. Increase the speed to high and beat for 5 minutes, scraping down the sides of the bowl once with a rubber spatula. Add the vanilla and beat for 1 more minute. The icing will still be a little warm.
Using a metal icing spatula, first spread a thin layer of frosting all over the cake, including the inside of the center opening, to glue down the crumbs. Then coat the cake with the remaining frosting. Decorate the top of the cake with the chocolate shavings. To serve, gently slice the cake with a serrated knife or an angel food comb.