Peel and quarter the potatoes and place in a pot of cold water with some salt. Boil, then simmer until soft.
Meanwhile quarter the leeks lengthwise and wash well to eliminate any soil or grit. Chop into approximately 3/8" (1 cm) pieces.
Melt the butter with the oil in a skillet and add the leeks. Cook over medium heat until the leeks are translucent and soft.
Add the chicken stock, some salt and pepper, simmer for a few more minutes. Puree half of the leek mixture in a blender and set aside with the remaining leeks.
Mash the potatoes with a little butter and the sour cream, followed by the leek puree. Stir in the leek pieces and then season and garnish if desired.