Melt butter in pot.
Add leeks.
Saute leeks slowly until glassy.
Add broth and potatoes.
Bring to boil.
Reduce to simmer and cover.
Cook until potatoes are soft, stirring frequently.
Remove from heat.
If desired, use potato masher to mash soup slightly. Leave chunks, rather than mashing all the potatoes.
Add milk, cream, and salt and pepper to taste.
Reheat gently. Do not boil.
Serve.