In a food processor fitted with the steel blade, process the walnuts until finely ground, about 25 seconds.

Using a sharp knife, split the vanilla bean in half lengthwise. Use the side of the knife to scrape the seeds out of the pod. Add them to the food processor along with the remaining ingredients. Process until the ingredients are thoroughly incorporated, about 30 seconds. The dough should hold together when pinched between your fingers.

Roll the dough into small balls. Roll in the chopped pistachios. Refrigerate in an airtight container for up to 1 week (or freeze for up to 1 month).