01 - Place the peeled potato chunks in a large pot and cover with cold, salted water. Bring to a boil over medium-high heat, then reduce to a simmer. Cook until potatoes are fork-tender, about 20 minutes.
02 - Drain the potatoes thoroughly and return them to the pot. Set over low heat for 1 to 2 minutes, stirring gently to evaporate excess moisture.
03 - Warm the milk and butter in a saucepan over low heat until the butter is fully melted.
04 - Use a potato masher or ricer to mash the potatoes until smooth and free of lumps.
05 - Add the warm milk and butter mixture, cream cheese, and sour cream to the mashed potatoes. Stir until the mixture is creamy and homogeneous.
06 - Incorporate kosher salt and black pepper into the mashed potatoes. Taste and adjust seasoning as preferred.
07 - Spread the mashed potatoes in a buttered 9x13-inch baking dish. Dot the surface with additional butter.
08 - Cover the dish and refrigerate for up to two days before baking.
09 - Preheat oven to 350°F. Cover potatoes with foil and bake for 30 to 40 minutes until heated throughout. For a browned top, remove foil for the last 10 minutes of baking.