In an ovenproof casserole or Dutch oven melt the butter and cook the leeks and onions until softened about 5 minutes. Stir in the potatoes and cook for 3-4 minutes. Bring the chicken stock to a boil and stir it into the vegetables. Cover and simmer for 20 minutes. PurÄe the soup in batches in a food processor of blender. Strain the soup. Stir in the cream, salt and pepper. Chill in the fridge. Just before serving, stir in some ice cubes. Serve the chives separately.
