Place russet potatoes in a large bowl; cover with water to 1 inch above potatoes.

Place Yukon Gold potatoes in a large saucepan; cover with water to 2 inches above potatoes. Bring to a boil over high heat. When water reaches a simmer, drain russet potatoes, and add to pan with Yukon Golds. Bring to a boil; reduce heat, and simmer 12 minutes or until potatoes are very tender. Drain.

Using a potato ricer, rice potatoes over pan. Stir in butter until melted. Gently stir in buttermilk, salt, and black pepper.

Variation: Butternut-Swirled Mashed Potatoes

Follow above recipe, reserving 2 tablespoons buttermilk. While potatoes simmer, bring 2 cups unsalted chicken stock, 2 tablespoons fresh sage leaves, and 1 (12 ounce) package peeled diced butternut squash to a simmer; cook 15 minutes. Reserve 2 tablespoons stock; drain squash. Discard sage. Place reserved 2 tablespoons buttermilk, 2 tablespoons stock, and squash in a blender; blend until smooth. Add squash mixture to mashed potatoes; swirl with a spoon.

Variation: Roasted Garlic and Parmesan Mashed Potatoes

Cut 1/2 inch off the top of 1 whole garlic head; drizzle with 1/4 teaspoon olive oil. Wrap in foil; bake at 400°F for 50 minutes or until very tender. Separate garlic cloves; squeeze to extract pulp. Discard skins. Follow above recipe. Stir in garlic pulp and 3/4 cup grated Parmesan cheese.

If making ahead, keep the potatoes warm by placing in a heatproof bowl, covering with plastic wrap, and setting over a saucepan of gently simmering water.