Cut and discard green top from leek; cut white portion into slices.
Melt butter in a Dutch oven over medium heat; add leek slices and saffron, and sauté 5 minutes or until leek slices are tender. Add white wine, and cook 1 to 2 minutes. Add squash and next 5 ingredients, and bring to a boil. Reduce heat, and simmer, uncovered, 20 minutes or until squash and carrots are tender. Remove from heat, and let cool slightly.
Process squash mixture, in batches, in a blender or food processor until smooth, stopping to scrape down sides. (A handheld immersion blender may also be used.)
Return squash mixture to Dutch oven. Add cream, salt, and pepper; simmer 10 to 15 minutes or until thickened. Garnish, if desired.