Smash potatoes chunky and rustic, or mash them silky smooth.
Preheat oven to 225°. Place potatoes in a large saucepan with cold water to cover by 2 inches. Bring to a boil over medium heat; boil 25 minutes or until potatoes are tender. Drain and spread potatoes in a parchment paper-lined jelly-roll pan. Bake 15 minutes.
Bring milk and butter to a simmer over medium heat. (Do not boil.)
Transfer potatoes to a large bowl. Add hot milk mixture; mash potatoes with a potato masher. Slowly add buttermilk, mashing to desired consistency. Add salt and pepper to taste.