Melt butter in a large saucepan over medium heat; add onion and celery, and sauté 10 minutes or until tender. Add garlic, and sauté 1 minute.
Stir in broth and next 3 ingredients. Bring to a boil; reduce heat, and simmer 15 to 20 minutes or until potato is tender. Remove and discard cinnamon stick. Process sweet potato mixture in a blender or food processor until smooth.
Return mixture to Dutch oven; stir in 1 1/2 cups whipping cream and 1/4 teaspoon salt. Cook over medium heat, stirring constantly, until thoroughly heated.
Beat 1/4 cup whipping cream, molasses, pinch of salt, and nutmeg until soft peaks form. Serve with soup, and garnish, if desired.
* 1 1/2 cups fat-free half-and-half may be sustituted for 1 1/2 cups whipping cream.