Use a good-quality
cocoa powder. You can use natural or Dutch-process for these, whichever one you like. Just remember that the chocolate flavor of the finished cookies is dependent on the quality of cocoa powder you use. So it’s worth using a decent one. I used Valrhona. See notes below on ingredients.
If you like extra-crisp biscotti, you can flip each one over midway during the second baking, in step #6. I sometimes smear one side of the cookies with melted dark chocolate. (And omit the sugar glaze.) When dipped in a warm espresso, I can’t imagine anything better.