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.