Cook pasta in a 6- to 8-quart pot of boiling salted water (see
Tips) according to package directions. Reserve 1 1/2 cups cooking water, then drain pasta in a colander and add to pancetta mixture in skillet. Add 1 cup reserved cooking water along with cheese and butter and cook, tossing constantly, over high heat until pasta is well coated (add more reserved water if necessary), about 1 minute. Add salt and pepper to taste and serve sprinkled with parsley and remaining tablespoon sage.