Preheat oven to 350 degrees.
Combine powdered sugar, flour, and butter in a food processor, mix until ingredients form a ball.
Dump ball onto counter and divide into 6 parts. Place a part of the dough into each individual 4 1/2" tart pan with removable bottom.
Push dough down along pan going up sides. Push into all the indention on side of pan. If you have a small rolling-pin (the kind for pies & pizzas) this works wonderfully.
Add about a spoon full of chopped pistachios to each tart and lightly press into dough.
Bake for 10-12 minutes, until lightly browned.
Cool completely before filling.