Have ready a bowl of ice and cold water. In a saucepan of boiling salted water blanch basil, a handful at a time, 2 seconds, transferring with a slotted spoon to bowl of ice water to stop cooking. Drain basil in a sieve and pat dry. In a food processor puree basil with remaining ingredients until smooth and season with salt and pepper. Pesto may be made 2 days ahead and chilled, its surface covered with plastic wrap.