Put the room-temperature butter in a mixing bowl and beat it lightly with a wooden spoon to soften some more.
Add the sugar and beat together with the butter. You can use an electric mixer if you want but it's not necessary. Just beat until combined and no more, we don't want to incorporate any air here.
Add the flour and mix until it starts to form clumps and looks like large chunky breadcrumbs. Ditch the spoon and use your hands to bring it together into a smooth dough.
Roughly chop the pistachios and add ⅔ of them into the shortbread dough. Reserve the other third of them to sprinkle on top later. Use your hands to knead the pistachios into the dough so they are evenly dispersed. Don't overwork the dough.
Shape the shortbread dough into a log shape, roughly the width of a paper towel roll. Wrap it in plastic wrap and twirl the edges like a Christmas cracker to seal it in. Roll it on your work surface to smooth out the edges and make it more round.
Chill the dough log in the fridge for one hour or until it feels very firm to the touch. You can also make the dough ahead of time and keep it in the fridge for up to 3 days before baking.
Once you are ready to bake, preheat the oven to 340°F / 170°C and line a cookie sheet with parchment paper.
Remove the shortbread dough from the fridge and unwrap it from the plastic wrap. Use a sharp knife to slice it into rounds about ½ an inch thick. Place the rounds on the prepared cookie sheet with enough room to spread.
Bake for 12-18 minutes or until the edges of the cookies are just starting to turn golden brown. Let cool on the baking sheet for 10 minutes before transferring to a cooling rack to cool completely.
Once the shortbread has cooled, chop up your dark chocolate and melt in the microwave in 30-second increments until totally melted.*
Dip the shortbread cookies into the chocolate to about halfway then lay them on a sheet of parchment paper. Sprinkle the remaining chopped pistachios on top and let them set. Optional: You can also sprinkle a tiny bit of sea salt on there if desired.
Notes
Make sure you use real chocolate and not chocolate chips - chocolate chips contain extra ingredients to make them bake stable that stops them from melting properly.*On melting the chocolate - For a more professional finish you can temper your chocolate (see reasons to do this in the blog post above). Did you know you can easily temper chocolate in the microwave without a thermometer? See my blog post on how to temper chocolate to find out how.