Preheat oven to 400˚F. Pierce each potato several times with fork. Bake for 55 to 60 minutes or until tender; let cool for 10 minutes.
Cut each potato in half lengthwise. Scoop out potatoes, leaving 1/4-inch border all around and keeping skins intact. In bowl, mash potato pulp until smooth; mix in 1/2 cup Parmesan, ricotta, pest, salt and pepper.
Divide potato mixture evenly among potato shells. Sprinkle with remaining Parmesan. Bake on parchment-lined baking sheet for 20 to 25 minutes or until heated through. Broil for 1 to 2 minutes or until tops are golden brown.