Melt butter in a large Dutch oven over medium-high heat; add leeks, and sauté 10 minutes or until tender. Add potato, 1/2 cup wine, and broth. Using a piece of kitchen twine, tie bay leaf, 2 thyme sprigs, and 2 parsley sprigs into a bundle; add to pan. Bring to a boil; reduce heat, and simmer, uncovered, 1 hour and 15 minutes or until potatoes are very tender. Discard herb bundle.
Process potato mixture, in batches, in a blender or food processor until smooth, stopping to scrape down sides. Return puree to Dutch oven. Add remaining 1/4 cup wine, cream, and next 3 ingredients. Garnish, if desired.