Chocolate Angel Food Cake

Ingredients

• 1 cup cocoa powder, divided
• 1 3/4 cup granulated sugar, divided
• 1 cup cake flour
• 2 teaspoons instant espresso
• 1/4 teaspoon salt
• 1/2 cup boiling water
• 1 tablespoon zest from 1 orange
• 15 large egg whites
• 3/4 teaspoon cream of tartar
• 1 orange, sliced thin for garnish

Directions

1. Move the oven rack to the lower-middle slot, then heat the oven to 350 degrees F. Into a medium bowl, sift together 1/2 cup cocoa, 3/4 cup sugar, and the flour, then put that bowl aside.
2. In a large bowl, combine the remaining cocoa with the espresso powder and salt. Pour in the boiling water, add the orange zest, and stir until the mixture is smooth; set it aside for now.
3. Fit a standing mixer with the whip attachment, add the egg whites, and beat them on medium speed until foamy. Add the cream of tartar and keep mixing until the whites start to look opaque, about 1 minute. Turn the mixer up to medium-high and gradually stream in the remaining sugar. Beat until the meringue holds a stiff peak but still looks glossy rather than dry, 3 to 5 minutes.
4. Whisk about 1 cup of the meringue into the cocoa-water mixture to loosen it. Gently fold in the rest of the whites in 3 stages. Sift 1/3 of the flour mixture over the meringue and fold it in with a light hand. Add the rest of the flour mixture in 2 additions, sifting and folding each time just until incorporated, being careful not to knock the air out of the egg whites.
5. Carefully transfer the batter into the pan with a spoon. Bake until the cake has risen, is barely set, and a toothpick poked into the center comes out clean, about 40 minutes.
6. Place the pan on a wire rack for 5 minutes, then turn the pan upside down onto the rack and leave the cake to cool completely in the inverted pan, about 4 hours. Slide a knife around the inside edge of the pan, turn the cake out onto a serving plate, and serve it with the orange slices.