I got this recipe 10 years ago from one of the old cooking BBS Fidonets. (remember those?!) It was posted by Eleanor Creighton. I hope she doesn't mind my re-posting it here. Use dried chantrelles, cepes, morels or a mixture.
In a small bowl combine dried mushrooms, 1/4 cup broth and 2 tbs. of the Madeira. Let stand 20 minutes stirring occasionally. Drain off liquid and discard.
Squeeze out excess liquid. Coarsely chop mushrooms. In a large saucepan melt butter. Add onion and cook until tender 2 to 3 minutes or until most liquid has evaporated. Stir in flour. Add 4 cups broth and pepper.
Bring to boiling stirring constantly. Reduce heat simmer uncovered, 30 minutes stirring occasionally. Remove 2 cups soup mixture and puree in blender or food processor.
Return to saucepan. Stir in buttermilk and remaining Madeira and heat through. Do not boil.