Finely grind all of the pistachios, then dump into a medium bowl. Add in the flour, starch, salt, and baking powder and whisk until combined. In a large bowl, using an electric mixer, beat together the eggs and sugar until lighter and fluffy (about a minute), then add in the oil, rose water (optional), and vanilla and beat until combined. Pour the dry ingredients into the large bowl with the egg mixture and stir together using a spatula. It will be stiff but not rollable - chilling the dough will make it less sticky and more manageable for rolling into balls.

Cover the large bowl with plastic wrap and chill in the fridge for at least six hours (this is the longest part of the recipe) - I recommend either making the dough in the morning and baking the cookies in the evening, or letting it chill overnight and making them the next day.

Once chilled, preheat the oven to 350° Fahrenheit and prep one or two cookies sheets. Grease the cookie sheets, line with parchment paper, then grease again with cooking spray or butter. Remove the chilled dough from the fridge immediately before rolling and baking the cookies (if it gets too warm it will be too sticky to handle). Pour your two cups of powdered sugar into a small bowl for rolling. Use a teaspoon or other small spoon to scoop out dough and use your hands to roll into 1-inch-diameter balls. Toss balls in the powdered sugar until thoroughly coated (the more powdered sugar on each ball, the better they'll look once baked!).

Place balls two inches apart on the cookie sheets (I usually can get 15-18 cookies on each sheet) - do NOT flatten the cookie dough balls. Once each sheet is full, bake for 13 minutes and then remove and let cool completely before eating or plating. Cookies will still be soft when you take them out of the oven but will firm up once cool. Repeat until all the dough has been used! You can make larger balls for larger cookies, just be sure to spread them apart farther on the cookie sheet.