Place butter in a small sauce pan and brown butter in a medium heat. Cook butter until it turns brown and has nutty aroma. About 8-10 minutes.
Place potato in a large Dutch oven. Add in water just enough to cover the potatoes. Add 1 teaspoon salt and cook the potato until tender.
When potato is ready, drain completely using a colander. Make sure that water is thoroughly drain by shaking the colander.
Return potato in the pan and cook in a medium heat and mixing until potato is dry.
Transfer potatoes in a big bowl and add salt ( start with 2 teaspoons), add half of the browned butter and add warmed buttermilk ( start with 1/4 cup) and mash potatoes until smooth. I prefer mine with little chunk in it.
Add in more buttermilk if the mash potato is to dry, add salt and season according to your liking.
Spoon into a bowl and drizzle with the remaining browned butter.
Garnish with green onion.