1. Sift cake flour and measure 1/2 cup. In a bowl, mix flour, 1/4 cup sugar, cocoa, instant espresso, and salt.
2. In a deep bowl (at least 4-qt.) with a mixer at high speed, whip egg whites and cream of tartar until they form a thick foam. Continue to beat and add remaining 1 cup sugar, 1 tablespoon about every 15 seconds, whipping until whites hold very stiff peaks. Mix in vanilla.
3. Sprinkle about 1/2 the flour mixture over whipped whites. Gently fold mixture until blended. Sprinkle with remaining flour mixture and gently fold to blend well. Scrape batter into a 10-inch tube cake pan with a removable rim; spread level.
4. Bake in a 350° oven until center of cake springs back when lightly pressed and begins to pull from pan side, 40 to 45 minutes.
5. Invert pan on a rack; let stand at least 1 hour. Run a thin knife between cake and pan rim. Push cake from pan rim. Slide thin knife between cake and pan bottom, and around tube to pan bottom. Invert cake onto a plate and lift out tube section. Drizzle mocha sauce onto top of cake, letting it run down the sides. Use an angel food cake cutter or serrated knife to cut slices.
Nutritional analysis per serving, plain: 155 cal., 12% (18 cal.) from fat, 4.8 g. protein; 2 g fat (1.1 g sat.); 32 g carbo (1.3 g fiber): 111 mg sodium; 0.4 mg chol.
Nutritional analysis per serving with sauce: 188 cal., 16% (31 cal.) from fat, 5.4 g. protein; 3.4 g fat (2 g sat.); 37 g carbo (1.6 g fiber): 111 mg sodium; 0.4 mg chol.