HEAT oven to 350°F.
COOK potatoes in boiling water in saucepan 15 to 20 min. or until tender; drain.
PLACE potatoes in large bowl. Add sour cream and VELVEETA; mash with potato masher. Add ham; mix lightly. Spoon into 13x9-inch baking dish sprayed with cooking spray; sprinkle with Parmesan.
BAKE 30 min. or until heated through.