This Italian basil and garlic sauce is one of the greatest delicacies of all time. It is most often used as a sauce for pasta, but my favorite way to savor it is to spread it on lightly toasted French bread and eat it as an appetizer. Traditionally it is pounded in a mortar, but this blender method works just as well and it is a lot quicker. Many contemporary recipes call for the addition of pine nuts, but I find them superfluous and indistinguishable in pesto. The addition of butter, however, is pure magic.
Note: To prepare pesto for freezing, don't add the cheese or butter. Freeze the basil, olive oil, and garlic purée in a tightly covered container for up to 6 months. Alternatively, freeze in ice cube trays, then place the cubes in a Ziploc bag. When ready to use, thoroughly defrost, then stir in the cheese and butter.
Put the basil, olive oil, and garlic in the container of a blender or food processor and purée until smooth. Turn off the machine and scrape down the sides as necessary.
Pour this mixture into a container and stir in the cheese. Stir in the soft butter and chill until ready to use. (The secret of a smooth pesto is to stir in the cheese and butter by hand — not machine.)
If you are going to use the pesto on pasta, then thin the pesto with a few tablespoons of boiling pasta water just before saucing the pasta. Pesto will keep for 1 month if covered with ¼ inch of olive oil and stored in the refrigerator.