Place the basil leaves, walnut pieces, garlic cloves, and olive oil into a food processor or strong blender.  Pulse until chopped, about 20 seconds.  Use a small spatula to scrape down the sides and pulse again, another 3-5 seconds.  Add the parmesan, pulse 3-5 more seconds until the pesto is your desired consistency.  Add salt and pepper and pulse again to taste (if needed).  Serve.