In the bowl of a food processor fitted with metal blade place basil, parsley, chopped garlic and pine nuts. Close lid and pulse off an on until coarsely combined but pieces of herbs are still visible.
Next, with processor running, slowly drizzle olive oil into basil mixture until mixture is completely smooth. You may need to scrape the bowl down once or twice.
Scrape pesto into a bowl and stir cheese in by hand.
Once cheese is added, stir in butter until completely combined.
Taste and add a little salt, if needed
Store pesto in the refrigerator in an airtight container for up to 5 days. If you'd like you can drizzle a thin layer of olive oil over the top of the pesto to help keep pesto from discoloring.
*See notes for longer storage.