Method
1. Add potato and kosher salt to a clean saucepan
2. Fill with water and cover
3. Heat until boiling
4. Simmer until tender - 10 min
5. Drain and reserve liquid
6. Transfer potatoes to large mixing bowl
7. Mash briefly with potato masher
8. Add buttermilk to a clean saucepan
9. Heat gently until lukewarm - low heat
10. Transfer milk mixture to potatoes
11. Add baking soda to the potatoes
12. Mash completely with potato masher until smooth
13. Mix in unsalted butter to the potatoes
14. Season with salt & pepper
15. Serve immediately