Each serving of creamy butternut squash soup is equal to one and one-third veggie servings.
Combine the squash and vegetable broth in a saucepan and bring to a boil over high heat. Reduce the heat to medium, and simmer for about 20 minutes or until the squash is very tender.
Let cool, and then purée the mixture in a blender. (You may prepare the recipe in advance up to this point; you can keep the mixture refrigerated in a covered container for up to 2 days.)
Return the purée to the saucepan and turn the heat to medium-low. Stir in the non-fat sour cream along with the butter, cayenne, and salt and pepper. Cook and stir until heated through (do not boil). Then taste and season, if necessary. Keep warm over low heat.
Meanwhile, heat skillet over moderately high heat until very hot. Spray with cooking spray, then sauté mushrooms, stirring, until golden brown, about 5 minutes. Serve the soup with sautéed mushrooms and chives.