6 medium potatoes, peeled and washed

4 Tablespoons butter

Wash carefully the leeks, trimming off all roots and darker green tough parts then slice into very thin discs. 

Melt butter in heavy bottomed pot.
Add leeks and cook gently over low medium heat until soft.
Cut potatoes into small chunks and stir into leeks, adding hot water to cover and cooking over medium heat until potatoes are cooked through.
Use immersion blender to purée until smooth.
Season with salt and pepper to taste and serve hot with a dollop of sour cream, if wanted.