First step, brown the butter! This isn’t a necessary step and it does add extra time, but it makes the cookies so super delicious and flavorful! To brown the butter, add the butter to a large skillet over medium heat. Let it melt and start to bubble. You want it to bubble (and you can occasionally whisk it) until you start to see brown bits on the bottom of the pan. Whisk it well then remove it from the heat. Pour the butter in a bowl and place it in the fridge to get cold. Ideally, you can do this the night before! It will need anywhere from an hour to 3 to 4 to fully harden.
To make the cookies, add the cold butter to a stand mixer and beat on medium speed. Beat in the brown sugar and granulated sugar and beat on medium speed until combined, about 3 to 4 minutes. Beat in the vanilla extract until combined.
Beat in the flour until it is just combined. Beat in the dark chocolate and chopped pistachios.
Remove the dough from the bowl and separate it into two pieces. Roll each piece in a log that is about 2 inches thick. Wrap it tightly in plastic wrap and refrigerate for at least two hours.
Preheat the oven to 350 degrees F.
After two hours, unroll the log and slice it into ½-inch slices. Slice down gently and if the nuts and chunks break the dough apart, you can push the pieces back together. You want rounds on the baking sheet about 2 inches apart.
Bake the cookies for 10 to 12 minutes, until they are slightly golden. Remove and sprinkle with a pinch of flaked salt. Let cool completely then serve!