Preheat oven to 375 degrees. Grease an 8-inch square (or similar size) baking dish.
In a large pot of boiling salted water cover and cook potatoes until completely tender, about 20 minutes. Drain and return to pot.
Mash the potatoes partly, then add the milk, 1/3 C of the chives, cream cheese, butter, egg, 1/2 C cheddar cheese, salt and pepper. Mash until smooth (or beat with a hand mixer until smooth).
Spoon mixture into baking dish and smooth the top. Sprinkle remaining 1/2 C cheddar cheese and remaining chives on top. (Make ahead: let cool 30 minutes then cover and refrigerate for up to 24 hours. Increase cooking time to 55 minutes).
Bake uncovered for 40 minutes, until it starts to brown and bubble, and the inside is hot.
Enjoy!