1. In a bowl set in hot water over low heat, beat egg yolks with cream and vinegar or lemon juice.
2. Continue beating the eggs with a whisk until they begin to thicken.
3. Gradually add butter and continue beating the sauce until it is thick.
4. Add lemon juice, salt, and cayenne pepper to taste.
5. Put the bowl in a pan of lukewarm water until serving time.
6. Serve with any hot or cold poached fish.