The secret to a perfect angel food cake lies in beating the egg whites and then stabilizing them with cream of tartar. Dutch-process cocoa has nothing to do with the Netherlands but refers to a process in which the cocoa is "dutched," or alkalized, resulting in a more mellow and full-bodied cocoa. Though the regular variety can be used, Dutch-process cocoa gives the sauce a subtle richness. Use an ungreased tube pan so the egg whites in the batter will cling to the sides and "climb" as the cake bakes.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, 3/4 cup sugar, cocoa, and cinnamon in a small bowl, stirring with a whisk. Beat egg whites in a large bowl with a mixer at high speed until foamy. Add cream of tartar and salt; beat until soft peaks form. Add 3/4 cup sugar, 2 tablespoons at a time, beating until stiff peaks form. Sift flour mixture over egg white mixture, 1/4 cup at a time, folding in each portion. Fold in vanilla, juice, and almond extract. Spoon the batter into an ungreased nonstick 10-inch tube pan, spreading evenly. Break air pockets by cutting through batter with a knife.
Bake at 325° for 50 minutes or until cake springs back when lightly touched. Invert pan, and cool completely. Loosen the cake from sides of pan using a narrow metal spatula. Invert the cake onto plate. Cut into 12 slices using a serrated knife. Serve with Dark Cocoa Sauce.