Bake potatoes until soft and cooked through, 425 for an hour. Remove from oven and cool to room temperature and refrigerate for several hours or overnight.
Peel chilled potatoes and cut into 1/2″ cubes.
Place cubes in a large mixing bowl and gently stir in Jack cheese, sour cream and melted butter. Season.
Turn into a sprayed casserole dish and bake for 30 minutes at 400.
Remove from oven and stir in Parmesan cheese.
Divide into individual baking dishes, if desired, and return to oven for another 15 minutes or until golden and bubbly. {Place smaller dishes onto a baking sheet} 8 servings.