Cream butter and sugar in a medium bowl (or bowl of your stand mixer). Keep beating until smooth, 1-2 minutes. Whisk in the egg, salt and rose water. Then toss in the pistachios and beat until incorporated. Lastly, stir in the flour and beat just until no spots of flour are left.
Place dough on a piece of plastic wrap and roll it into a log. (It’ll be soft, so you can chill it in the fridge and roll it again 30 minutes later.) Wrap the log in aluminum foil (double it for extra support) and chill in the refrigerator for at least 3 hours or up to 3 days.
When you are ready to bake the cookies, preheat the oven 350° F (180°C). Line a baking sheet with parchment paper. Unwrap the cookie log. Using a sharp knife, slice log into cookies, about 1/2 inch (1-cm) thick. Place cookies on the baking sheet, leaving about 1-inch (2.5-cm) space between them.
Bake cookies for 10-15 minutes, until they’re golden brown and set (their tops should be dry, not wet, but paler in color than the edges).Bigger thicker cookies need about 15 minutes, while smaller (or thinner) cookies need 10-12 minutes.
Remove cookies from the oven and let cool. Store in an airtight container for up to 5 days.