1) Chop the leeks into small pieces.
2) In a large saucepan over medium heat, melt the butter. Add the leeks and a heavy pinch of salt and sweat for five minutes. Decrease the heat to medium-low and cook until the leeks are tender, approximately 25 minutes, stirring occasionally.
3) Add the potatoes and the vegetable stock, increase the heat to medium-high and bring to a boil. Reduce the heat to low, cover and gently simmer until the potatoes are soft, approximately 45 minutes.
4) Turn off the heat and puree the mixture with a hand blender until smooth. Stir in the double cream, buttermilk and white pepper. Taste and adjust seasoning if desired.
Sprinkle with chives and serve immediately, or chill and serve cold.