Place the first amount of pistachios in a bowl of a food processor and process until finely ground.
Add the flou and sugar and process to incoporate.
Add the butter and process until you have a soft dough. It should not be sticky, but if it is, that's ok.
Roll the dough into a 10 inch cylinder, then place it on top of a large pieces of parchment paper, rolling it as you smooth it from one end to the other. The dough will stretch to about 12 inches as you roll it and smooth it.
Refrigerate for at least one hour, or up to three days. Until the dough is cold enough to slice.
Preheat your oven to 350F and line one large baking sheet with parchment paper.
Remove the dough from the oven and brush it with an egg white, or water.
Roll the dough in the chopped pistachios, then slice it to 1/2-inch thick cookies.
Line the cookies oven the prepared baking sheet and bake for 15-18 minutes over the middle oven rack.
Remove from the oven and allow the cookies to chill and set in the pan for at least 30 minutes before removing them. When out of the oven the cookies are very soft and will break and crumble if moved.