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.
Make it ahead: Prepare 1 day ahead; cover and refrigerate. Reheat before serving.
Mashed Potatoes with Bacon and Crispy Scallions: Prepare recipe as directed; keep potatoes warm. Cook 4 bacon slices in a medium skillet over medium-high until crisp. Drain on paper towels, reserving drippings in skillet. Crumble bacon. Cook 1 cup sliced scallions in hot drippings until lightly browned and crispy, 3 to 4 minutes. Transfer to paper towels to drain. Top warm potatoes with scallions and bacon.
Rustic Mashed Red Potatoes with Parmesan: Omit crème fraîche. Substitute 3 lb. small, unpeeled red potatoes, roughly chopped, for russet potatoes. Prepare recipe as directed, adding 4 oz. herbed cream cheese (about 1/2 cup) and 4 oz. Parmesan cheese, shredded (about 1 cup), to potatoes before coarsely mashing. Top with 1/4 cup chopped fresh chives.