Discard roots and green tops of leeks. Cut leeks in half lengthwise; rinse well, and chop.
Melt butter in Dutch oven over medium heat; add leeks, and sauté 4 minutes or until tender. Stir in squash and next 4 ingredients; bring to a boil. Cover, reduce heat, and simmer 40 minutes or until squash is tender. Remove from heat, and cool 10 minutes.
Process squash mixture, in batches, in a food processor or blender until smooth, stopping to scrape down sides. Return mixture to Dutch oven; stir in whipping cream and milk, and simmer, stirring occasionally, 20 minutes or until thoroughly heated. (Do not boil.) Serve with desired toppings.