Preheat Oven to 350℉
Roast Pistachios: Chop and roast Pistachios on parchment paper at 350℉ for approx 8 minutes. Remove and cool. Set aside.
In a medium bowl, whisk together the flour and cornstarch and set aside.
Using a stand or hand mixer, cream the butter approx 2 to 3 minutes, until light and fluffy.
Mix in powdered sugar, and beat on medium speed until incorporated.
Take bowl off of the stand mixer if you are using one, and add the flour and cornstarch mixture all at once, using a spoon to combine. When the flour mixture is almost combined, add in the the pistachio's. Work together to bring to a solid ball of dough, being careful not to over mix. Over mixed shortbread dough will develop gluten and make the finished product tough, not tender. (If the dough doesn't come together, you can add up to a tablespoon of water)
Divide dough into 2 disks and wrap in cling wrap. I chill in the fridge for at least 20 min.
Dust surface with powdered sugar and roll out dough to about 1/2 inch.