Remove papery outer skin of garlic bulb. Rub with olive oil and place on a small baking sheet. Bake in preheated oven for 45 minutes, or until skin can be easily pierced with a fork.
While garlic is roasting, peel and quarter the potatoes. Place in a large pot and cover with water. Bring to a boil and allow potatoes to simmer until very soft, about 20 minutes. Drain water and mash potatoes with butter until smooth and fluffy.
Squeeze softened garlic cloves out of skin into pot; in a small bowl, mix wasabi powder with just enough water to form a thick paste. Mash garlic and wasabi paste into potatoes, pour in milk and continue to mash until mixture is light and fluffy; season with salt and pepper to taste and serve hot.