Place basil leaves, garlic and pine nuts(walnuts) in a food processor and pulse it several times.
Scrape down everything and add parmesan. Give couple of pulse.
With food processor running, slowly emulsify olive oil in slow stream. This will help keep the olive oil from separating.
Stir in salt and pepper to taste.
To store, refrigerate in a clean airtight container or toss it fresh on hot paste.