Clean and peel mushrooms and heat them in melted butter. Stir in flour, add stock and bring to the boil, stirring well to prevent lumps. Allow to cook slowly for 20-30 minutes. Rub through a wire sieve and return to the saucepan. Beat egg yolks with a little milk, season and add to soup with the remainder of milk. Do not allow to boil after adding egg yolks otherwise they will curdle. Serve with toast.