Melt the butter in a large pot over medium heat. Add the chopped leeks with a pinch of salt and pepper. Cook, stirring occasionally, until softened. Slide in the garlic, rosemary, thyme and bay leaf. Sprinkle in the cinnamon, nutmeg, Ancho chile powder and cayenne or Piment d’Espelette. Cook, stirring, until fragrant. Add the sweet potatoes and another pinch of salt and pepper. Cook for a few minutes, stirring, to coat the sweet potatoes in the spices. Add chicken broth and/or water (I usually do a mix of chicken broth and water, but plain water works just fine) to cover the potatoes by 1-2 inches. Bring to a boil, then reduce to a simmer and cook, partially covered, until the sweet potatoes are tender.

Remove the bay leaf and puree the soup in a blender (or you can use a hand 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.

Serve the soup with a sprinkle of toasted chopped pecans, or with a few toasted pumpkin seeds. Do Ahead: The soup can refrigerated for up to 5 days or frozen for up to 2 months. If needed, thin with a splash of water or stock before serving.