Melt the butter in a large pot over medium heat. Add the leek, and season with salt and pepper. Cook, stirring occasionally, until softened, about 3 minutes.
Stir in the garlic, cinnamon, chili powder, nutmeg and bay leaf. Cook, stirring, until fragrant, about 30 seconds. Slide in the sweet potatoes, and season with more salt and pepper. Stir the potatoes to coat in the spices. Pour in the broth and water. Bring to a boil, then reduce the heat to a simmer and cook until the sweet potatoes are tender, about 10 minutes.
Remove the bay leaf and puree the soup in a blender (or you can use a stick blender). Add more water or broth to reach the consistency you want (I like it thick, but still soup-like). Taste and season with additional salt, pepper and/or spices as needed.
If you’d like, drizzle a bit of heavy cream or coconut cream over the soup, and sprinkle with chopped pecans or pumpkin seeds.