Preheat the oven to 325 degrees F. Line a baking sheet with parchment paper.
In a stand mixer with the paddle attachment, cream the butter, confectioners sugar and vanilla for several minutes until light and fluffy.
Sift the all purpose flour and salt together.
Turn the mixer speed down to low and slowly add the pistachio flour followed by the all purpose flour. Scrape down the sides once or twice to make sure the dough is evenly mixed.
Use a tablespoon-sized scoop to measure out cookies. Use your hands to roll each cookie into a ball.
Chill the cookies for 15 minutes and then bake for 20 minutes. While the cookies are baking, sift together the topping ingredients in a bowl.
Remove the cookies from the oven. While they're still warm, carefully coat each cookie in the sugar mixture and set aside to cool. Once the cookies have cooled (about 15-20 minutes), toss them again in the sugar mixture.
Notes
You can order pistachio flour online, if it's not available in your local store. Almond flour, which is available in most grocery stores, may be substituted for the pistachio flour. You can also making your own pistachio flour by grinding roasted unsalted pistachios in a food processor.