This recipe puts store-bought pesto so badly to shame that you should make it only if you're going to commit to continuing to make it.
Fill a large sink with water and rinse the basil well, then pull of the leaves, spin dry in a salad spinner, and measure out 2 packed cups (if you're a little short, don't worry too much).
In a food processor, whir together the basil, pine nuts, garlic, salt. Then, with the machine running, drizzle the olive oil through the feed tube and process until completely creamy-looking.
If you're using the pesto now, stir in the cheese and butter, but if you're freezing it, don't. To use it, put the pesto in a large serving bowl and stir in a couple of tablespoons of pasta-cooking water, then stir in the drained pasta and serve with more cheese for passing.
If you're freezing it, divide the pesto into four small plastic containers (we use empty yogurt containers) and freeze solid, then pop the pesto out of the containers and store in a Ziploc freezer bag onto which you've added this note: "Thaw, then add a couple tablespoons of pasta cooking water along with 1 tablespoon of butter and lots of freshly ground parmesan before stirring into pasta." Makes enough to sauce 2 pounds of pasta.