INSTRUCTIONS
[Make the pesto]
1. In the bowl of a food processor, combine the basil, garlic, oil, pine nuts, the 1 tsp. salt and several grindings of pepper. Process until a coarse paste forms, stopping occasionally to scrape down the sides of the bowl with a rubber spatula. Transfer to a bowl and stir in the 1 ½ cups cheese and the butter.
[Cook the pasta]
2. Meanwhile, bring a large pot of water to a boil over high heat. Add the 2 Tbs. salt and the pasta to the boiling water. Cook, stirring occasionally to prevent sticking, until al dente, according to the package instructions. Drain, reserving about ½ cup of the cooking water.
3. In a serving bowl, mix 2 Tbs. of the cooking water with 1 cup of the pesto. (Store the rest for another use; see note above.) Add the drained pasta and toss to combine. Add as much of the remaining cooking water as needed to loosen the sauce. Serve, passing the cheese at the table.