Preheat the oven to 350F. Line a large baking sheet with parchment paper.
In a medium mixing bowl, whisk together the flour, cocoa, baking soda, and salt.
In a large mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter on medium speed until creamy. Add the sugar and vanilla and beat until well combined, about 2 minutes. Add the eggs, one at a time, stopping to scrape down the bowl between each, and beat until well combined.
Add the flour mixture and chocolate chips and mix on low speed just until combined. Scrape the dough out of the bowl onto a well-floured surface. Flouring your hands and the dough as needed (it will be sticky), shape the dough into a uniform ball and divide in half. Shape each half into short logs and brush off any excess flour.
Place the logs on the baking sheet several inches apart and press each into a rectangle about 1-inch thick and 2½ inches wide. (They will spread during baking so give plenty of space between them.)
Bake for 30 to 35 minutes or until firm to the touch. Let cool on the baking sheet for 5 minutes or until just cool enough to handle but still very warm. Using a sharp serrated knife, cut each log diagonally into ¾-inch wide slices. Flip the slices cut side down on the baking sheet.
Bake for another 10 to 15 minutes or until crisp. Transfer to a wire rack to cool completely. Store biscotti in an airtight container for up to 1 week at room temperature or freeze tightly wrapped in a freezer container for up to 1 month.
Notes
I highly recommend using a scale to measure your flour as it’s the most accurate method and will prevent you from ending up with dry biscottis. If you don’t have a scale, fluff your flour with a spoon and spoon it into your cups before leveling it off with a knife. This method prevents you from overpacking the measuring cup.
Just like the flour, I recommend using a scale for the cocoa powder as well. Too much cocoa powder will also make your biscotti taste bitter instead of tasting rich and decadent.
Room temperature eggs will mix more evenly into the mixture. Place your eggs into a bowl of warm water for a couple of minutes. Doing so will quickly bring them to room temperature if you forget to take them out of your fridge.
Feel free to replace half of the chocolate chips with sliced almonds, walnuts, white chocolate, or even dried fruit such as dried cherries or cranberries.
For a sugary top, brush the shaped logs lightly with an egg wash (1 egg beaten with 1 teaspoon water) and sprinkle with coarse sugar before baking.
For a more tender cookie, you can cut down on the baking time after you’ve sliced them.
To make larger biscotti, slice the cookies at a larger angle and increase the baking time by a couple of minutes. Keep an eye on them as they bake!
Keep in mind that the chocolate biscotti will continue to harden as they cool.