In a food processor, combine the ½ cup cashews and 3 tablespoons of coconut milk. Process until almost smooth, about 1 minute, scraping the bowl as needed. Add the ginger, garlic, lime zest, 2 chili halves, 1 tablespoon of sugar and 2 teaspoons fish sauce; process until finely chopped, 1 to 2 minutes. Transfer to a medium bowl. Add the shrimp, rubbing to coat them thoroughly. Marinate at room temperature while you make the sauce and prepare the grill.