The only way to have true Genoese "pesto" is to go to the Liguria region of Italy where the tiny fragrant sweet basil for which the Italian Riviera is famous can be found. Locally grown basil, although not quite the same, will produce a perfectly acceptable alternative for those of us who may find it impractical to fly to the Riviera whenever we feel like having "pesto".
Put the basil leaves, olive oil, pine nuts, garlic, and 2 teaspoon of salt into a food processor or blender and grind until fine and almost creamy. You can prepare the sauce ahead of time up to this point, and refrigerate or even freeze it. Cover the surface with olive oil to prevent the basil turning black.
When ready to use it, transfer the sauce mixture to a large bowl and stir in the two grated cheeses.
Bring 4 quarts of water to a boil in a large saucepan or pot. Add 1 tablespoon of salt and the pasta, stir well and cook until "al dente". Drain and toss with the sauce, 2 tablespoons of hot water, and the butter.
From: Giuliano Hazan
The Classic Pasta Cookbook
1993, London and New York, Dorling Kindersley
Used with permission