Place dried mushrooms in a small bowl. Add the 1/2 cup broth and the wine. Let stand 20 minutes. Drain and discard excess liquid. Coarsely chop mushrooms; set aside.
Remove 4 cups soup mixture, if desired; puree in blender or food processor. Return to saucepan. Stir in buttermilk and heat through. Do not boil. Makes 8 to 12 servings.
Prepare soup through pureeing step; cover and refrigerate up to 1 day. To serve, stir in buttermilk and heat through.