Cocoa adds an extra flavor dimension to a classic angel food cake. Serve this accompanied with a scoop of Chocolate Ice Cream, Chocolate Sorbet, or drizzled with Hot Fudge Sauce. Fresh fruit is also a good accompaniment. The cake travels well, too, so pack some slices in with your next picnic.
Makes one 10 X 3 ½-inch cake, 14 to 16 servings
Position a rack in the center of the oven and preheat the oven to 325°F.
In a 1-quart bowl, thoroughly blend the flour with the cocoa powder, salt, and ¾ cup of the superfine sugar. Set this mixture aside. Place the remaining ¾ cup superfine sugar in a measuring cup near the mixer.
In the grease-free bowl of an electric stand mixer using the wire whip or in a mixing bowl using a hand-held mixer, whip the egg whites on low speed until they are slightly frothy. Add the cream of tartar and whip the egg whites until they begin to mound. While the egg whites are whipping on medium speed, slowly sprinkle on the remaining ¾ cup of superfine sugar, 2 tablespoons at a time, then continue whipping the whites until they are firm, but not dry. Blend in the vanilla, then remove the bowl from the mixer.
Sprinkle the dry ingredients over the whipped egg whites, 3 tablespoons at a time and gently fold them into the whites, using a long-handled rubber spatula.
Turn the batter into a 10 X 4-inch tube pan, preferably with a removable bottom. Use the rubber spatula to smooth and even the top. Tap the pan on the countertop gently a few times to eliminate any air bubbles.
Bake the cake in the preheated oven until it is golden brown, springs back when lightly touched, and a cake tester inserted near the center comes out clean, about 40 minutes. Remove the pan from the oven and immediately invert it onto its feet, or hang it by the center tube over a funnel or the neck of a bottle. Leave the cake to hang for several hours, until it is completely cool.
To remove the cake from the pan, run a thin-bladed knife around the inside of the pan and around the tube. Gently loosen the cake from the edges and push the bottom of the pan up, away from the sides. Run the knife between the bottom of the cake and the bottom of the pan and invert the cake onto a plate, then reinvert, so it is right side up. Angel food cake is best cut with a serrated knife using a sawing motion.
The cake will keep at room temperature, well wrapped in plastic, for 3 days, or it can be frozen for up to 3 weeks. If frozen, defrost in the refrigerator for 24 hours before serving.