In a large bowl, or a stand mixer mix together the softened butter...
...all the powdered sugar...
Zest the orange and set it aside.
Cream the butter, sugar, and salt together until it's light and fluffy then add in the egg.
Mix the egg in well. When it's fully incorporated, sift together the cocoa and flour in a separate bowl
Add in the mixed cocoa and flour...
...and the orange zest.
When it's blended together, fold in the pistachios by hand. The dough is stiff, so that's why you want to add the pistachios by hand.
On a lightly floured surface, or a silicone mat, roll the dough into a cylinder about 10 inches long and 2 inches across.
I broke this dough into about 3 cylinders. Some for baking now and some for baking later.
Wrap the cylinders in waxed paper and pop them into the fridge to chill.
They can stay refrigerated there for several days before you bake, and I imagine one could also freeze the cylinders as long as they're tightly wrapped since this is basically a butter cookie.
Set your oven rack to the upper and lower middle positions and preheat it to 375 degrees. Line a couple of cookie sheets with parchment paper, or silicone mats.
When the oven is up to speed and the dough is nice and firm, unwrap and slice. Not too thick but just right.
Put the sliced cookie dough on the baking sheet giving them enough room between, (about 1 inch) and pop them into the oven.
Bake them for about 12 minutes, or until the tops of the cookies seem dry. Take them out of the oven, off the cookie sheet and set them on a rack to cool.
When they're nice and cool serve them up!
These cookies are amazing, and the orange zest adds just the right note of contrast with the chocolate. Plus they really, really, do keep well in the fridge for later baking.