Chop leeks into small pieces.

In a 6-quart saucepan over medium heat melt the butter. Add the leeks and salt, sweat for 5 min. Decrease heat to medium low and cook until tender. Approx 25 min, stirring occasionally.

Add the potatoes and vegetable broth, increase to medium high heat and bring to boil. Reduce heat to low, cover and gently simmer until potatoes are soft, approx. 45 min.

Turn off the heat and puree with a immersion blender until smooth.

Stir in heavy cream, buttermilk and white pepper.

Serve hot or cold.