In a large stock pot, melt butter and saute leeks for 3 minutes. Add potatoes, stock and salt and bring to a boil. Reduce heat, cover, and simmer for 30 minutes, until the potatoes are tender.

Add the milk, caraway seeds, dill and pepper. Let simmer another 15 to 20 minutes; the soup should become thick and the potatoes should begin to fall apart. Stir in cheeses and allow them to melt, then stir in the yogurt and heat through, but do not boil. Adjust seasoning and serve hot, topped with parsley.