Cut potatoes in half if small, or in 1 1/2" pieces depending on size. Bring water to boil in a medium sauce pan, when it is boiling add 1/4 cup salt. Add potatoes to pot and return it to a slow boil, just above a simmer. Cook potatoes at a slow boil until tender. Remove from heat and drain in a colander. Place potatoes in a bowl and cover loosely and set aside.
Just before serving, using an immersion blender, stir and mash potatoes, gradually stirring in soy milk until you've reached the desired consistency. Stir in butter and taste for seasoning, adding additional salt to taste and plenty of black pepper. Serve immediately.