1. In a large pot or Dutch oven, heat the butter or olive oil over medium heat.
2. Add the slices leeks and season with salt. Cook the leeks over moderate heat for 5-7 minutes, stirring frequently, until they’re completely soft and wilted.
3. Add the thyme, if using, and chile powder, and stir for about 30 seconds, cooking them with the leeks to release their flavor flavors.
4. Pour in the water, and add the potatoes and bay leaf.
5. Cover and simmer until the potatoes are tender when poked with a sharp knife. Depending on which potatoes you used, it could take anywhere from 15 to 30 minutes.
6. Pluck out the bay leaves and puree the soup with the white pepper, seasoning with more salt if necessary. I use an immersion (stick) blender, but if you use a standard blender, be sure not to fill it more than half-full and secure the lid, and cover it with a tea towel when blending, to avoid hot soup or steam for causing problems. Don’t use a food processor as that will make the potato purée gummy.
If the soup is too thick, add a bit more water, until it’s the desired consistency.