Melt butter in large pot over medium heat and cook onions until softened, about 5 minutes.

Add broth, 2 cups water, carrots and potatoes. Once boiling, reduce heat to simmer, halfway covered until veggies are tender, about 25 minutes. Remove from heat and let soup cool.

Puree soup in a blender or food processor in batches until smooth. Put all of soup back in pot and season with cayenne pepper, salt and pepper to taste.

Serve warm or cold. Garnish with cream and chives.