How to Make It
Place potatoes in a large Dutch oven with cold water to cover by 2 inches. Bring to a boil over medium-high. Boil until tender, 25 minutes. Drain; return potatoes to Dutch oven over medium. Cook, stirring once, until potatoes dry out slightly, 50 seconds.
Place buttermilk and softened butter in a microwavable glass bowl; microwave on HIGH until warm, 30 to 60 seconds. Add warm buttermilk mixture, crème fraîche, salt, and white pepper to potatoes; mash with a potato masher.
Spoon potatoes into a 4- to 6-qt. slow cooker. Cover and cook on HIGH until heated through, about 2 hours. Or reheat in a tightly covered baking dish at 350°F until heated through, about 1 hour.