Add the butter and sugar to the bowl of a stand mixer fitted with a paddle attachment and beat until creamy, about 2 minutes.
In another medium mixing bowl whisk together the flour, cocoa powder, orange zest and salt together until combined. With the mixer on low, slowly add the flour mixture to the butter mixture, mixing until evenly combined.
Place an 18-inch piece of plastic wrap on a clean workspace. Sprinkle with the chopped pistachios then turn the dough out onto the plastic and use your hands to shape it into a 12-inch log with the pistachios on the outside like a crust.
Wrap the dough tightly in the plastic wrap, pressing the pistachios in place and refrigerate for at least 2 hours or overnight.
When ready to bake the cookies, preheat the oven to 325ºF (170ºC). Unwrap the cookie dough and slice into ½-inch slices. Arrange on a parchment paper-lined baking sheet, spaced 2-inches apart.
Bake until crisp, about 16 minutes. Transfer to a cooling rack until completely cool and store in an airtight container.