Prick the sweet potatoes several times with a fork. Microwave on High for 7-10 minutes until just tender. Let cool slightly before handling.
In a large saucepan or Dutch oven, heat the oil over medium-high heat. Add the onion and cook, stirring occasionally, for 2-4 minutes until it just begins to brown. Stir in the garlic and cook for 1 additional minute.
Add the vegetable juice, green chiles, ginger, and allspice to the pan. Bring the mixture to a gentle boil and cook for 10 minutes to let the flavors meld.
Peel the sweet potatoes once cool, then chop into bite-sized pieces. Add half of the chopped sweet potatoes to the pan.
Place the remaining sweet potato pieces in a blender or food processor along with the vegetable broth and peanut butter. Blend until completely smooth and creamy.
Pour the blended mixture back into the soup pot, stirring thoroughly to combine. If the soup is too thick, thin it with water to reach your desired consistency. Season with pepper to taste.
Heat the soup until hot, then ladle into bowls. Garnish with chopped cilantro if desired, and serve warm.