Grease a large baking dish. Preheat the oven to 175 degrees. Peel the potatoes and cook them in salted water until soft, drain and mash very finely in a large bowl. Mix in the cream cheese, crème fraîche and milk until you get a very smooth mixture. Season well with salt, pepper and a pinch of garlic salt. Pour into the form, cover with flakes of butter and bake for about 30 minutes, until the mixture is loose and light brown.