Preheat oven to 350°F with the rack in the lower middle position. Line a half sheet (18”x13”) pan with parchment paper or non-stick silicone baking mat. The latter makes it easier for scraping/shaping the biscotti dough using a spatula.
Mix together and sift flour, cocoa, salt and baking soda in a medium bowl. Set aside.
Cream the butter, brown and white sugar together using a hand or stand mixer at medium speed for 3 to 5 minutes. Dissolve the espresso granules in the vanilla extract, then add to the mixture. Add the eggs one at a time and mix at medium-low speed until evenly incorporated. Add the flour mixture in quarter portions and mix at low speed until the dough has no patches of dry flour left. Lastly, still at low speed, pour the chocolate chips and mix until evenly distributed; about 15 seconds. You may hear some chocolate chips breaking, but that's alright. Turn off and scrape the sides and give it a quick stir.
The dough will be very sticky at this point. Do not attempt to scoop and shape with your hands unless you like a goopy mess. Instead, using a stiff non-stick spatula, transfer half the dough onto the lined pan on one side. With the same spatula, and perhaps another one, patiently scrape and shape into a log along the length of the pan. Make it two inches wide, leaving about 2” from both the short ends of the pan. Do the same for the other half of the dough. You will end up with two long logs on the pan with about 2.5" in between and 2" from each edge.
Bake in the preheated oven for about 30 minutes. The dough will spread considerably and there will be a few cracks on the top of two large cookie logs after baking. The frenshly baked cookie will bounce back when pressed lightly with your fingertips.
Remove from oven and cool on the pan for 10 minutes before transferring to a cooling rack to cool completely (about 1 to 1.5 hours, depending on the ambient temperature).
Preheat oven to 325°F. Place one of the cookie logs on a cutting board and cut into 1/2-inch slices using a sharp serrated knife (a sharp bread knife will work well for the purpose). Place the slices cut side down on the same lined baking sheet. Bake for 15 minutes and transfer on a cooling rack to completely cool. Each log will use up the entire sheet and you will have to bake the slices in two batches.
Want snappier and drier cookies? Turn off the oven and leave them in for another 2-3 minutes before cooling.
These cookies will last for a week in an airtight container, but I doubt it. The original recipe said these can be frozen for up to 3 months, but I have not successfully had any leftovers to freeze.