Peel and cut the potatoes into 2-inch cubes
Add the potatoes to a large pot. Add enough water to just cover.
Bring to a boil, then simmer until potatoes are fork tender. Drain the potatoes.
In a small pan, mix together the butter and the buttermilk. Just heat to a bare simmer.
Run the potatoes through a food mill or potato ricer if you have either or use a potato masher to mash them.
Slowly add in the warm buttermilk mixture. Stop and stir, you may not need all the buttermilk, so never add it all at once.
Season to taste. Add chopped green onions if using.