Put the potatoes in a pot of cold water, bring to the boil and cook at a rolling boil until the potates are cooked through. Peel the potatoes while they are still hot and put them back in the pan over a low heat for a couple of minutes to dry them.
At the same time warm the butter and milk together until the butter is just melting. Use a potato masher to mash the potatoes thoroughly with the butter and milk.
Season well with the salt and a little pepper .Make a paste with the wasabi powder and a little water or milk then stir it into the mash. Check taste and add more wasabi if needed.