Makes 18 servings.
Pack the basil leaves very firmly into a 4 cup measuring cup. Be firm — lean on it. The garlic will sterilize any dirt that comes with the leaves. If you feel that you must wash the leaves, do it ahead of time and spread them out to dry (but just long enough to get rid of surface moisture — the basil must be fresh for full flavour).
Cram the leaves into a food processor intermixed with all ingredients. If there is not enough room, leave the cheese; after the processor has been running for a few minutes, the level in there will drop sufficiently to allow you to add the cheese. Continue to run until you have a fine green paste and there are no chunks of garlic. Mixing will warm up the paste. Quit mixing before paste gets hot.
The cubes in a standard ice cube tray are just right for 1 person servings. After freezing, remove cubes of paste from tray and put in freezer bag.
For each person being served you will need (for 4 people, multiply everything by 4):
Pesto is often served cold. This may be because three of the ingredients (basil, garlic & butter) are heat sensitive. We like it warm.
Bring salted water to a boil and add pasta. Cooking will take 20 minutes.
The following should be done very slowly to avoid over heating. Count on 25 minutes. Put butter on low heat. Clarified butter does not taste as good as cloudy butter so keep the butter in motion until it is just barely melted and then add (frozen) cubes of basil paste and continue to stir until they are melted and warm, but not hot — both basil and garlic lose 90% of their flavour at cooking temperatures. Turn off heat and leave until pasta is ready.
Drain pasta and stir in basil paste. Keep mixing until all the rotini is the same shade of green. Serve immediately. If you let it sit around, it will still be edible in 15 minutes but it won't be nearly as good. Sprinkle pinenuts and parmesan to taste. Be liberal with the pinenuts — they are expensive by the pound, but cheap by the serving and they add protein, texture and flavour. Some recipes call for pinenuts in the paste. We like them much better as a garnish.