Preheat oven to 375°. Scrub sweet potatoes; pierce each several times with a fork. Place in a foil-lined 15x10x1-in. baking pan; bake until tender, 1 to 1-1/4 hours.
While the sweet potatoes are baking, put a small sauté pan over a medium flame and heat the oil with one of the rosemary sprigs. Add the pancetta and cook until crisp, about 5 minutes. Set aside.
Cut a thin slice off the top of each potato; discard slice. Scoop out pulp, leaving thin shells.
In a large bowl, mash pulp with butter, salt and pepper; stir in cheese, pancetta and the remaining rosemary sprig, chopped. Spoon into potato shells.
Return to pan. Bake until heated through, 15-20 minutes.