Preheat oven to 400°. Combine first 3 ingredients in a large bowl, tossing to coat. Place on a large rimmed baking sheet. Bake 45 minutes; remove garlic, and set aside. Return potatoes to oven, and bake 30 minutes or until potatoes are tender.
Let potatoes stand 15 minutes or until cool enough to handle. Peel potatoes, discarding skins, and transfer to a large bowl. Squeeze garlic pulp from cloves into bowl with potatoes.
Add cream and 1/4 cup hot water to potatoes and garlic. Beat with an electric mixer 1 minute or until light and fluffy. Stir in butter; season with kosher salt and black pepper to taste.