Place potatoes in a large pan with salted water to cover, over high heat and cook until tender. Drain and force potatoes through a strainer or food mill while still hot into a saucepan. Gradually stir in milk, sour cream and butter and mix well until puree is soft.
If necessary, add more milk. Season with salt and pepper, cover saucepan and separate.
Saute bacon in a skillet until crisp. Drain in paper towels and stir into mashed potatoes.
Spread half of the potatoes in an ovenproof casserole and top with 1 cup of the shredded cheese. Spread remaining puree on top and cover with remaining cheese. Bake in preheated oven to 375°F (190°C) until golden.
Garnish with chopped chives.
More delicious purees:
Sweet Potato Puree with caramelized apples
Yellow potato purée with leeks and thyme