Method
1. Add unsalted butter to a clean large saucepan
2. Melt gently until completely melted - 149°C, medium heat
3. Set aside and add leek and kosher salt to the large saucepan
4. Return to the heat and heat gently until tender while stirring occasionally - 20 min, 135°C, low heat
5. Add potato and vegetable stock
6. Heat until boiling - 174°C, medium heat
7. Simmer - 45 min, 149°C, low heat
8. Let cool - approx 1 min
9. Use stick blender until smooth
10. Add buttermilk bit by bit while mixing
11. Add white pepper
12. Add heavy cream bit by bit while mixing
13. Season to taste and garnish with fresh chives
14. Serve immediately