Stir the instant espresso into the boiling water until dissolved. Add half of the cocoa powder and whisk until smooth and glossy. Set aside to cool slightly.

In a separate bowl, sift the cake flour with the remaining cocoa powder and half of the granulated sugar. This removes lumps and keeps the cake light.

Pour the egg whites into a large clean mixing bowl. Add the cream of tartar and salt. Beat on medium speed until soft peaks begin to form. Gradually add the remaining sugar and continue beating until the whites form glossy, firm peaks. Stir in the orange zest.

Gently fold the cooled cocoa-espresso paste into the whipped egg whites, taking care not to deflate them.

Sift the flour mixture over the batter in two or three additions, folding gently each time. Work slowly until everything is incorporated without knocking out the air.

Spoon the batter into an ungreased tube pan and smooth the top. Bake at 350°F (175°C) for 35–40 minutes, or until the top springs back when lightly touched. Turn the cake upside down on the pan’s legs or onto a bottle and let it cool completely before removing.