Preheat the oven to 150 degrees C and put the rack in the top third of the oven.
Line a couple of baking trays with parchment paper.
In a large mixing bowl beat together the butter and the sugars until smooth, then add the egg, vanilla extract and baking soda and mix together.
Combine the flour and salt, then add them into the bowl and mix together.
Roughly break/chop the pistachio nuts and dark chocolate, then stir into the bowl.
Using a tablespoon, scoop balls of dough onto the lined baking trays leaving a good 10 cm between each cookie as they will expand out in the oven!
Bake the cookies for about 15-18 minutes or until a light golden colour then remove from the oven. After 5 mins remove from the baking tray and allow to cool on a wire rack. You can sprinkle extra pistachios/chocolate/sea salt on at this point before the cookies have completely cooled. They will keep for 3-4 days in an airtight container.