Peel the potatoes and cut into cubes. Place them in a large saucepan. Cover with cold, salted water and bring to a simmer over medium heat. Cook for 20 minutes, or until soft.
Drain the potatoes and mash them, using a potato masher, together with the hot milk, until smooth.
Add the miso paste, butter, and soy sauce. Season to taste with ground garlic powder and pepper.
Sprinkle the mash with chopped spring onion and some soy sauce.
