Preheat oven to 425 degrees. Scrub potatoes with a brush; pat dry. Prick in several places, and bake on a baking sheet for 40 to 60 minutes, or until tender. Cut a 1-inch slice from the top of each potato, lengthwise; discard skin from slice and place pulp in a bowl. Using a spoon, carefully scoop the pulp from each potato, leaving a thin shell; add pulp to the bowl. With an electric mixer on low speed or a potato masher, beat or mash potato pulp; add sour cream or yogurt, herbs, and Parmesan cheese, and beat or mash until smooth (If necessary, stir in 1 to 2 tbsp. milk to make desired consistency). Stuff potato mixture into shells. Sprinkle 1 tbsp. mozzarella on each potato. Place in a shallow baking dish and bake for 15 to 20 minutes, or until lightly browned.