Bring a large pot of water to boil over HIGH heat. Add potatoes, cover, and return to a boil. Reduce heat and simmer for 15 to 20 minutes, or until tender all the way through when pierced with the tip of a sharp knife. Drain well.

Add cream cheese, sour cream, about half of the milk, and the butter and mix with an electric mixer just until combined. Add as much of the remaining milk as needed to reach the desired consistency and mix again until no lumps remain but being careful not to overmix. Add seasoned salt, Havarti, and cheddar and stir with a spoon until well incorporated. Season to taste with additional salt (if needed) and fresh ground black pepper.

Transfer potatoes to prepared baking dish and cover with foil. Bake immediately or refrigerate for up to two days.

Remove dish from refrigerator about an hour before baking time to take the chill off. Preheat oven to 350 degrees F and bake, covered for 30 to 45 minutes, or until warmed through. Garnish with chives and serve.