Preheat oven to 325°F. Line a baking sheet with a silicone baking mat or parchment.
In a medium bowl, sift together flour, cocoa, baking powder and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat together butter and sugar on medium speed until light and fluffy, about 5 minutes.
Add eggs, one at a time, beating on medium speed to incorporate thoroughly before adding the next.
Add the flour mixture in two additions, mixing on low just long enough to incorporate between additions.
Fold in pistachios.
Cover and refrigerate dough for 15 minutes.
Remove dough and roll into two logs about 12 inches long. Place on prepared baking sheet about 3 inches apart. Cover and freeze for 15 minutes.
Bake for 40 minutes, rotating the baking sheet halfway through. Let cool.
Slice the loaves at a 45 degree angle and return the biscotti to the prepared baking sheets.
Bake for 45 minutes, flipping the biscotti halfway through baking so they dry completely.