Preheat oven to 350 degrees. Line baking sheet with parchment paper or a non-stick cooking mat; set aside.

In a food processor fitted with the blade attachment, add the shelled pistachios. Process until a fine crumb forms. Remove a tablespoon of the pistachios for topping the cookies and set the rest aside.

In a mixing bowl, use a handheld mixer to beat together the sugar and egg until light and fluffy. Add in the butter and vanilla and mix until well combined. Pour in the chopped pistachios, flour, and baking powder, and mix until just combined.

Use a small cookie scoop or spoon to measure out cookie dough on to prepared pan. Sprinkle the tops of the cookies with the leftover chopped pistachios.

Bake for about 10-12 minutes, or until cookies just start to turn color. Let the cookies cool on the baking sheet for five minutes before moving to a cookie rack to cool completely.