How to Make It
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.
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, black pepper, garlic pulp, and Parmesan cheese.