3 cups sweet potato, peeled and cut into 1-inch chunks
1 cup leeks, white parts only, cut into 1/2-inch pieces
5 cups whole milk (or 4 2/3 cups 1% milk and 1/3 cup cream)
2 bay leaves
2 teaspoons salt
1/8 teaspoon cayenne pepper
Combine all ingredients in a large saucepan and simmer, stirring regularly to prevent scorching, until sweet potatoes are tender, about 45 minutes. Pull out the bay leaves. Blend until completely smooth. Serve at once, or chill and reheat.
Note: I used my immersion blender and it did a pretty good job, although it was a bit stringy. I don't know if that was because I used frozen leeks, or if a standard blender would have been better. The soup still tasted good.