Step 1Saute leeks and onion in butter in large pot over medium heat until softened but not browned, 10 to 12 minutes.
Step 2Add potatoes, broth, salt and cayenne. Cover and simmer gently until potatoes are tender, 30 to 45 minutes. Process through food mill or press through fine strainer using wooden spoon, forcing as much vegetable pulp through as possible.
Step 3Cool to room temperature; add cream. Chill several hours or overnight. When ready to serve, garnish with finely chopped chives.