Chocolate Angel Food Cake

Heat the oven. Tweak oven rack to lower now middle position and heat oven to 175 c 175 c sieve 120 ml cocoa 180 ml now sugar and flour into a medium bowl rest aside set now remaining cocoa espresso powder now and salt in large bowl mix in boiling water and now orange zest until smooth rest aside in a standing now mixer fitted with whip attachment mix egg whites on medium speed until frothy tip in cream of now tartar and keep to mix until whites are beginning to turn opaque about now 1 minute increase speed to medium high and slowly tip in remaining cup sugar keep to mix until meringue forms stiff but not now dry peak 3 to 5 minutes now beat about 240 ml meringue into cocoa and water now mixture to lighten see notes in 3 stages gently turn in in remaining whites sieve 1 3 flour mixture now onto meringue and gently turn in in in 2 additions sieve and turn in now remaining flour mixture into batter until mixed taking care not to deflate egg whites now gently spoon batter into pan see notes cook in the oven until cake is puffed just rest and toothpick now inserted into center comes now out clean about 40 minutes rest pan on rack for 5 minutes now then invert pan onto rack so that cake can completely leave to go cold in pan upside down about now 4 hours see notes run knife now along inside of pan and now invert cake onto serving plate to bring to the table bring to the table with orange slices.