Homemade Chocolate-Glazed Angel Food Cake

Preheat oven to 375°F (190°C). For Cake: Sift flour, cocoa and 3/4 cup granulated sugar in medium bowl. Beat egg whites, cream of tartar, vanilla extract and salt in large mixer bowl until soft peaks form. With mixer running, gradually beat in remaining granulated sugar until stiff peaks form. Gradually fold in flour mixture, one-third at a time. Spoon batter into ungreased 10-inch tube pan with removable bottom. Bake for 35 to 38 minutes or until cake springs back when touched. Cool by turning the cake (in the pan) upside down, propping on top of a bottle until it cools to room temperature. Run knife around the cake to loosen, then invert onto a serving plate. For Glaze: Combine powdered sugar, cocoa and water in medium bowl. Stir until smooth. Drizzle glaze over top of cake, letting it drip down naturally. Let glaze set for 30 minutes or until hard. Serve with vanilla frozen yogurt, if desired.