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1
Adjust oven rack to lower-middle position and preheat oven to 350u00b0F. Sift 1/2 cup cocoa, 3/4 cup sugar, and flour into medium bowl; set aside.
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2
Place remaining cocoa, espresso powder, and salt in large bowl. Stir in boiling water and orange zest until smooth; set aside.
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3
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.
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4
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.
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5
Gently spoon batter into pan. Bake until cake is puffed, just set, and toothpick inserted into center comes out clean, about 40 minutes.
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6
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.
