Preheat oven to 300℉/150℃ and line a baking sheet with parchment paper or a silicone mat.
Blend roasted shelled pistachios with a food processor until finely ground. Set aside.
In a large bowl, add butter and sift in confectioners' sugar. Using a mixer or spatula, mix them until combined. Add vanilla extract. Mix until well combined.
Sift in all-purpose flour and salt. Add finely ground pistachios and mix with a spatula until well incorporated.
Scoop 1 1/2 teaspoon of dough and roll into balls. Arrange the balls onto the prepared pan.
Bake the cookies for 16-18 minutes. Remove from the oven and cool the cookies slightly.
While the cookies are still warm, roll them in confectioners' sugar.
Cool the cookies completely before transferring them into an airtight container.
Notes
Make sure to measure the flour correctly if using volume measurement. The cookies will end up too dry if you are adding too much flour. Use a spoon to scoop the flour into the measuring cup. Then, level the top with a knife or other straight-edged utensil.