Make the cranberry puree: In a heavy saucepan combine the cranberries, the Port, and the sugar and simmer the mixture, stirring occasionally, for 7 to 10 minutes, or until the cranberries burst and the mixture starts to thicken. In a food processor puree the mixture and force the puree through a fine sieve into a bowl, discarding the solids. The puree keeps, covered and chilled, for 3 days.
Make the soup: In a large heavy saucepan cook the onion and the carrots in the butter over moderately low heat, stirring occasionally, until the onion is softened, add mace, ginger, 1/2 teaspoon of white pepper, squash, sweet potatoes, and 4 cups of the broth, and simmer the mixture, covered, for 30 minutes, or until the vegetables are very soft. In a blender or food processor puree the soup in batches, transferring it as it is pureed to a saucepan, and stir in the remaining 2 cups broth, the additional white pepper, and salt to taste. The soup keeps, covered and chilled, for 1 day.
To serve the soup: Reheat the cranberry puree and spoon it into a pastry bag fitted with a small plain tip. (Alternatively, spoon the puree into a small resealable plastic bag and cut off the tip of one corner.) Reheat the soup, divide it among soup bowls, and pipe about 1 tablespoon of the cranberry puree decoratively onto each serving.