Place potatoes, garlic and salt in large saucepan, and cover with cold water. Bring to a boil. Reduce heat, cover and simmer for 20-25 minutes or until the potatoes are done (when a fork can easily be inserted into them). Drain well.
Add the butter, milk, wasabi paste, and soy sauce. Use a potato masher to mash well and combine. Then with a wooden spoon, mix in scallions and continue mixing to your desired consistency. Add salt and pepper to taste.