Chop up the pistachios into fine pieces, or pulse them in a food processor a few times until you have small pieces.
In a small mixing bowl, melt the butter. Mix in the brown sugar, cinnamon, and pistachios.
Once the dough has finished rising, lightly punch it down to remove any excess air in it. Transfer the dough to a clean and lightly floured surface, then roll it out to a 12″ x 16″ (30 x 40 cm) rectangle. If the dough keeps springing back a lot while rolling it out, let it rest for 5 minutes before continuing.
Carefully spread the pistachio filling over the entirety of the dough. Cut the dough into 6 long strips, then roll them up tightly.
Transfer the rolls to a lightly buttered and/or lined eighth sheet pan*** Lightly cover the pan with a towel, and let the rolls rise in a warm environment until doubled in size (~30 minutes).
Preheat the oven to 375°F / 190°C.
(Optional, for browning) Make an egg wash by whisking together an egg with 1 tbsp. milk, then brush it over the rolls.
Bake for ~20 minutes, until the tops of the rolls are golden brown and the internal temperature of the rolls has reached 190°F / 88°C.