Cover the potatoes with cold water by 1 inch in a large pot and add the cooking salt. Bring to a boil, then simmer for 18 to 20 minutes until tender; drain and steam dry for 2 minutes.
Grease a 9-by-13-inch metal baking pan with 1 tablespoon butter. Mash the potatoes by hand, then fold in cream cheese, sour cream, milk, 2 tablespoons butter, seasoning salt and pepper.
Spread in the pan and make shallow ridges on top. Dot with the remaining 2 tablespoons butter; cool briefly and refrigerate within 2 hours, covering once chilled, for up to 24 hours.
Heat the oven to 350°F. Cover the cold metal pan tightly with foil and bake for 35 minutes.
Uncover and bake for 10 to 15 minutes more until the center reaches 165°F. Let stand for 5 minutes before serving.