For the "classic" Pesto recipe, I consulted Marcella Hazan and my two favorite Vegetarian cookbook authors, Mollie Katzen (Moosewood fame) and Deborah Madison (Vegetarian Cooking for Everyone). I was sure that Marcella's would be the best and certainly the most authentic recipe but I am going with a combination of the 3. I'll use asterisks to tell you how they differed in case you want to go with the Italian master. The directions are Hazan's which are clearly the best.
2 plump garlic cloves
3 Tablespoons pine nuts*
3 cups loosely packed basil leaves, stems removed**
1/2 cup freshly grated Parmesan, preferably Parmigiano-Reggiano
2-3 Tablespoons grated pecorino Romano to taste***
2 Tablespoons soft butter, optional
1/2 cup extra virgin olive oil
1 1/2 pounds pasta
1) Briefly soak and wash the basil in cold water, and gently pat it thoroughly dry with paper towels.
2) Put the basil, olive oil, pine nuts, chopped garlic, and an ample pinch of salt in the processor bowl, and process to a uniform, creamy consistency.
3) Transfer to a bowl, and mix in the two grated cheeses by hand. It is worth the slight effort to do it by hand to obtain the notably superior texture it produces. When the cheese has been evenly amalgamated with the other ingredients, mix in the softened butter, distributing it uniformly into the sauce.
4) When spooning the pesto over pasta, dilute it slightly with a tablespoon or two of the hot water in which the pasta was cooked.
*Katzen called for a full 1/2 cup - way too much but also suggested that walnuts, almonds, or a combination of all 3 could be used.
** Hazan called for only 2 cups. Katzen called for an additional 3/4 cups fresh chopped parsley. Genius.
*** Mollie Katzen totally left this out.