Chocolate Angel Food Cake Recipe

Adjust oven rack to lower-middle position and preheat oven to 350u0b0F. Sift 1/2 cup cocoa, 3/4 cup sugar, and flour into medium bowl; set aside.
Place remaining cocoa, espresso powder, and salt in large bowl. Stir in boiling water and orange zest until smooth; set aside.
In standing mixer fitted with whip attachment, beat egg whites on medium speed until frothy. Add cream of tartar and continue to beat until whites are beginning to turn opaque, about 1 minute. Increase speed to medium-high and slowly add remaining cup sugar. Continue to beat until meringue forms stiff but not dry peak, 3 to 5 minutes.
Whisk about 1 cup meringue into cocoa and water mixture to lighten. In three stages, gently fold in remaining whites. Sift 1/3 flour mixture onto meringue and gently fold in. In two additions, sift and fold remaining flour mixture into batter until combined, taking cake not to deflate egg whites.
Gently spoon batter into pan. Bake until cake is puffed, just set, and toothpick inserted into center comes out clean, about 40 minutes.
Set pan on wire rack for 5 minutes, then invert pan onto wire rack so that cake can completely cool in pan upside-down, about 4 hours. Run knife along inside of pan and invert cake onto serving plate to serve. Serve with orange slices.