1. Heat oven to 425 degree F. In a large bowl, toss carrots, squash, onion, and ginger with olive oil. Transfer to a jelly-roll pan in an even layer. Roast until vegetables are tender, about 40 minutes, stirring every 10 minutes.
2. Transfer vegetables to a large saucepan. Add broth, water, cream, salt, and cayenne pepper and bring to a boil. Reduce heat to medium-low and let simmer 10 minutes. Stir in butter. Puree soup in batches in food processor until smooth. Top each serving with sour cream and chopped cilantro, if desired. Makes 7-1/2 cups.