Scrub and pierce sweet potato; place on a microwave-safe plate. Microwave, uncovered, on high for 10-12 minutes or until tender, turning once. When cool enough to handle, remove and discard skin. Place potato in a large bowl and mash with milk.
Cook pasta according to package directions. In a large skillet, saute pine nuts and sage in butter until pine nuts are toasted. Remove from pan and set aside.
In the same skillet, saute garlic in oil for 1 minute. Stir in the sweet potato mixture, salt and pepper. Drain pasta, reserving 3/4 cup cooking water; add to sweet potato mixture. Stir in the cheese, pine nut mixture and reserved cooking water. Sprinkle with additional cheese if desired.