In a Dutch oven or soup pan, heat the oil over medium heat. Add the onions and leeks and cook until soft, about 5 minutes. Add the garlic and chilies and cook for another minute. Add the sweet potatoes, chicken broth, and Sunbutter, raise heat and bring to a boil. Reduce the heat, cover the pan and let simmer until the sweet potatoes are tender, about 20 minutes.
Blend in batches in the blender until smooth. Season to taste with salt and pepper.
Serve garnished with cilantro leaves and Sriracha.