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; 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, herbs, and Parmesan cheese, and beat or mash until smooth (If necessary, stir in 1 to 2 tablespoons milk to make desired consistency).
Stuff potato mixture into shells.
Sprinkle 1 tablespoon mozzarella on to each potato.
Place in a shallow baking dish and bake for 15 to 20 minutes, or until lightly browned.