Preheat the oven to 350 degrees F. Generously butter a 4-quart baking dish.
Peel and cut the potatoes into chunks. Bring a large pot of water to a simmer and add the potatoes. Bring to a boil and cook until fork tender, 20 to 25 minutes.
Drain the potatoes in a large colander. Place them back into the dry pot and put the pot on the stove. Mash the potatoes over low heat, allowing all the steam to escape before adding in all the other ingredients.
Turn off the heat and add 1 1/2 sticks butter, the cream cheese, heavy cream, seasoned salt, kosher salt and pepper. Mash to combine.
Spread the potatoes in the prepared baking dish. Throw pats of the remaining butter over the top of the potatoes and bake until the butter is melted and the potatoes are warmed through, 20 to 30 minutes.