Preheat the oven to 325 degrees. Line two baking sheets with Silpat mats or parchment paper.

Roughly chop ¼ cup of the pistachios and reserve for garnish. Place the remaining 2 cups pistachios in a food processor fitted with a steel blade. Process the pistachios until they turn into a paste, this will take 2 or 3 minutes, scrape down the sides of the bowl a few times during the processing.

Add 1 cup sugar to the pistachio paste and process in long pulses until combined. Add the egg whites, vanilla and food coloring if using and pulse to combine. You should have a thick batter. Using a small ice cream scoop or two teaspoons, drop the cookies onto the prepared pan, spacing two inches apart. Sprinkle the tops of the cookies with a little granulated sugar and the reserved chopped pistachios. Gently press the pistachios into the cookies.

Bake for 12 – 14 minutes for chewy cookies, 15 – 17 minutes for crispy cookies. The cookies should be lightly browned around the edges. Let cool on the pan for 10 minutes then remove to a cooling rack to finish cooling.