Preheat the oven to 350°F (180°C). Line a baking sheet with parchment paper.
In a large bowl, whip the sugar and eggs until homogeneous. Incorporate the flour, baking powder, cocoa powder, and zest.
Knead the dough on a floured work surface until it is smooth. Halve the dough and create a 12-in cylinder with each half. Place the cylinders on the baking sheet and bake for 30 min.
Let the dough cylinders cool 10 min on the plaque. Lower the oven to 300ºF (150ºC).
With a bread knife, cut the cylinders into diagonal, 1/3-in thick slices. Place the slices on ungreased baking sheets, taking care that they do not touch.
Bake the biscotti for another 20 min. Once they’re dry and crunchy, take them out of the oven and transfer them to cooling racks.
The biscotti keep for up to a month in a sealed container.
These biscotti are the ideal holiday cookies, since they can be baked ahead of time and in bulk quantities. Try dunking them in a glass of aperol spritz for a fully decadent experience.