Add potato cubes to a large pot and fill with water until covered by at least an inch.
Bring to a boil and cook for about 15 minutes. Drain the potatoes. Add the potatoes to a large bowl, along with sour cream, cream cheese, half and half, garlic salt, and pepper then mash using a potato masher.
Transfer potatoes to a lightly greased 9×13 inch casserole dish, cover with foil, and refrigerate for up to 2 days.
When ready to bake, remove potatoes from the fridge and let sit on the counter for at least 30 minutes.
Bake, covered at 325℉ for about 45 minutes or until heated through. Lightly toss potatoes and top with a pat of butter and any more salt and pepper and serve warm.