In saucepan, cook peeled chestnuts with 1 C broth, rind, and salt for 20 minutes.
Grind mixture to a puree.
Combine puree with rest of broth, cream, and ginger.
Bring soup to a boil, stirring occasionally.
Gradually stir 1 C soup into egg yolk, then return to rest of soup stirring constantly and bring to a boil, stirring occasionally.
Add sherry and reheat but don't bring to a boil.