Preheat oven to 350 degrees F. Line non-stick baking sheets with parchment paper or use the Silpat sheets to prevent the cookies from sticking.
In a food processor, pulse the pistachios until finely ground, but not pasty, approximately 30 seconds; set aside.
In a large bowl using your electric mixer, beat egg whites until stiff peaks form. Check out How To Make A Perfect Meringue.
A little at a time, slowly pour sugar over the stiff eggs whites and fold in with a rubber spatula until fully incorporated. Add the pistachios and fold in until fully incorporated.
Drop batter by tablespoonfuls onto non-stick baking sheets (1-inch apart).
Bake approximately 12 to 15 minutes until golden brown around the edges; remove from oven and let cool 15 to 20 minutes before removing from the baking pans to prevent cookies from breaking.
Yields 12 to 18 cookies.
Flourless Pistachio Cookies Recipe: https://whatscookingamerica.net/Cookie/PistachioMeringueCookies.htm