Heat the butter in a 4-quart saucepan over medium-high heat, then add the onion and carrots. Cook for 5-7 minutes until the carrots are almost tender. Add the broth, star anise and salt, bring to a boil, reduce the heat to medium-low and simmer, covered, for 15 minutes. Discard the star anise.Use an immersion blender or food processor to puree the soup until smooth. Add the heavy cream, white pepper and dill. Heat through and serve hot garnished with a sprig of fresh dill.