Put stock - which should be home made and reasonably clear, and not oversalted, in a saucepan and start heating gently while you wash the dried cepes (porcini in Italian) in a sieve.
Add the cepes to the stock and when boiling, turn down to a very gentle simmer. Don't cover, and allow to simmer until reduced to 2/3 volume.
Strain and allow to settle, as the cepes always give out a little sand, no matter how carefully you wash them. When ready to serve, decant from the sediment, reheat and season. Add the optional madeira and serve.