In a blender, combine the basil leaves, toasted pistachios, Parmesan cheese, garlic, 1/4 teaspoon salt and 1/8 teaspoon pepper. Blend for 15 seconds, and then with the blender running, slowly stream in the olive oil. Continue blending the pesto, scraping down the sides as needed, until it is puréed. Taste and season the pesto with additional salt and pepper as desired. Set the pesto aside.