Bring pot with diced potatoes to a boil, add a pinch of salt, reduce to a simmer, and cook for 20 minutes or until potatoes are tender. Drain in colander and return to pot. Mash with a potato masher or fork. Add milk, half of the butter, and half of the wasabi (to taste). Taste and add more if desired. Season to taste with salt. Wasabi can be potent, so taste the potatoes, and let sit on your tongue for a bit before deciding if you want to add more.