Process the pistachios in a good-quality food processor for 10–15 minutes, scraping down the sides every two to three minutes, until smooth. Add the oil and continue to process until extremely smooth and loose.

In a medium-sized pot, heat the egg yolks, sugar, salt, potato starch, water, and 1/4 cup of the pistachio mixture. Whisk well, then continue to stir until the mixture easily coats the back of a spoon, about 15 minutes. Allow to cool, then add it to the food processor and pulse to combine the custard with the pistachio paste.

Transfer to a ziplock freezer bag or container and freeze overnight. Once frozen, blend in the food processor until creamy. Freeze until serving.