Put peeled and quartered potatoes in a large pot, cover with cold water, and bring to a boil over medium-high heat. Reduce to a simmer and cook until fork tender, about 10 to 15 minutes. Drain and put potatoes in a large mixing bowl. With a handheld mixer on low speed, break up potatoes. Add sour cream, butter, 1/4 cup buttermilk, and 2 teaspoons prepared wasabi and whip potatoes on medium speed. Add more buttermilk until the desired consistency is reached. Season with salt and white pepper, to taste. Garnish with the remaining wasabi and green onions.