Chocolate Angel Food Cake

1/4 cup plus 3 tablespoons unsweetened cocoa powder
1 1/2 cups (about 12) large egg whites, at room temperature
1/2 teaspoon salt
1 teaspoon cream of tartar
1 1/2 cups superfine granulated sugar
1 teaspoon vanilla extract
1 1/2 cups sifted cake flour
2 ounces semisweet chocolate, grated

In a small bowl, dissolve 1/4 cup plus 1 tablespoon of the cocoa powder in 1/4 cup boiling water.

In a very large bowl, beat the egg whites with salt and cream of tartar until they hold soft peaks. Beat in 3/4 cup sugar, a little at a time, and beat whites until they hold stiff peaks. Fold in cocoa mixture and vanilla extract.

Into a bowl, sift together 4 times flour, remaining sugar and cocoa. Sift the mixture over egg white mixture in 2 batches, just folding it in each time. Fold in grated chocolate. Spoon batter into a 10-inch angel food cake pan and, with a knife, cut through several times to eliminate air pockets.

Bake in preheated 350 degree F oven for 40 minutes.

Invert in pan on rack and cool completely. Run a thin knife around cake and turn out on cake plate.