Melt butter in large pan over medium heat.Add leeks and onions, sauté until tender.
Add potatoes, chicken broth, chicken base, salt and pepper.
Bring to a boil then reduce heat to low.
Cover and simmer until potatoes are tender.
At this point you can take a potato masher and mash potatoes slightly.
Stir in milk. Cook until heated through. Don not boil.
Serve in soup bowls with parsley and parmesan cheese.