Heat the oven to 350°F and grease a 9-by-13-inch baking dish.

Place the potato chunks in a large pot and cover them with cold salted water. Bring the water to a boil and cook the potatoes for 15–20 minutes, or until they are fork-tender.

Drain the potatoes thoroughly, then return them to the warm pot. Mash with 6 tablespoons of butter, the softened cream cheese, sour cream, warm milk, salt, and black pepper until smooth, mixing only as much as needed.

Spread half of the mashed potatoes evenly in the prepared baking dish. Use the back of a spoon or an offset spatula to create a level layer.

Sprinkle 1 cup of cheddar cheese over the potatoes, followed by half of the crumbled bacon and half of the sliced green onions. Distribute the fillings evenly so every serving has a little of each topping.

Spread the remaining mashed potatoes over the filling layer. Finish with the remaining 1 cup cheddar, the remaining bacon, the remaining green onions, and the remaining 2 tablespoons butter cut into small pieces.

Cover the dish loosely with foil and bake for 25–30 minutes. Remove the foil and continue baking for another 10–15 minutes, until the casserole is hot and the cheese is melted and lightly browned.

Let the casserole rest for 5–10 minutes before serving. This short rest helps the layers settle and makes it easier to spoon neat portions.