In a large sauce pan, heat butter and olive oil over a medium heat. Saute leeks 3 or 4 minutes. Then cover and "sweat". When you "sweat" onions, you are letting them cook and soften, rather than caramelize and brown.
Add the chopped potatoes.
Add chicken broth.
Cover again and cook until the potatoes are soft when you poke a fork into them, about 20 minutes.
Use an immersion blender or puree the soup in batches in a blender. If you use an immersion blender, you'll get a chunkier, more rustic soup. Or puree it in a blender, for a more elegant soup.
If it's too thick, add more chicken broth or some milk or cream.
Season with salt and white pepper.