In a small bowl, stir together the sour cream and lime zest. Set aside in the refrigerator to allow the flavors to blend.
Melt butter in a large pot over medium heat. Add onion and garlic, and cook for about 5 minutes, until softened. Add sweet potatoes, and chicken stock. Season with cumin, chili flakes and ginger. Bring to a boil. Reduce heat to low, cover, and simmer for 15 minutes, until potatoes are tender.
Puree the soup using an immersion blender or regular blender. If using a counter top blender, puree in small batches, filling the blender just a bit past half way to avoid spillage. Whisk peanut butter into the soup, and heat through. Stir in lime juice, and salt.
Ladle into warm bowls, and top with a dollop of the reserved sour cream, a few pieces of diced tomato, and a sprinkle of cilantro.
