Saute the onion and celery in the butter until softened, then add the broth, squash, cumin, cilantro, thyme and jalapeno. Simmer until the squash is fully softened.
Puree the soup until smooth, then return to the pot. Add the corn and simmer until the corn is cooked. Season to taste.
Plate, garnish with the yogurt or sour cream, and serve.