Golden Mashed Potatoes

4 pounds Yukon Gold potatoes, peeled and quarteredKosher salt3 tablespoons unsalted butter, at room temperature 2/3 cup nonfat sour cream, at room temperature 2/3 cup buttermilk, at room temperature1 teaspoon table saltFresh-ground black pepper, to tastePlace potatoes in a 6-quart or larger saucepan and cover with cold water 1 inch above the potatoes. Add 1 heaping tablespoon kosher salt and place over high heat. Bring to a boil, reduce heat and simmer until potatoes can easily be pierced by dinner fork tines, about 17-19 minutes. (Take care not to overcook potatoes.)Drain potatoes well, place in work bowl of an electric stand mixer and mix at lowest speed to break up potatoes. Add butter and mix for 1 minute to distribute. Add sour cream, buttermilk, 1 teaspoon salt and 1/2 teaspoon black pepper, or to taste, and mix until combined. Do not overmix. Taste and adjust seasoning. Serve immediately.Serves 12.Nutrition values per serving: 180 calories (16.3 percent from fat), 3.3 g fat (1.9 g saturated fat), 34.3 g carbohydrate, 3.1 g fiber, 2 g protein, 10 mg cholesterol, 567 mg sodium.LeanNote: Use leftover buttermilk to make low-fat, lower-calorie buttermilk pancakes for a weekend breakfast.