Preheat the oven to 425 degrees F.
Trim off the top 1/2-inch of the garlic bulb with a sharp knife to expose some of the clove ends. Drizzle with olive oil, wrap loosely in aluminum foil, and place on a small baking sheet. Roast until the bulb is soft and fragrant, 45 to 55 minutes. Squeeze the cloves from the skins and set aside in a small bowl until ready to use.
Place the potatoes in a large stockpot and cover with cold water. Bring to a boil over high heat and simmer until a fork inserted into a potato releases easily, about 20 minutes.
In a small glass bowl, mix the wasabi powder with 1/2 teaspoon of water to start, adding any additional water by drops until a thick paste is created. Cover with plastic wrap and set aside to bloom, at least 5 minutes.
Drain water from the potatoes and return the potatoes to the pot, allowing excess water to evaporate. In a small saucepan or in the microwave, warm the butter and heavy cream. Add the mixture to the potatoes, along with the wasabi and roasted garlic cloves, and mash until smooth and fluffy. Adjust seasoning with additional salt and pepper, if needed.