Put sugar, egg, yolks, and salt into top part of a double boiler. Set into bottom part of double boiler and be sure water does not touch the top part. Heat and beat. Use a whisk or hand beater. Mixture will triple in volume. Let it get very hot. Heat the cream to hot.
When mixture is hot and thick, gradually add the cream and continue to beat. Cook a few minutes. Remove from heat, add vanilla, and mix. Pour around apple mold, or serve with any dessert that needs a sauce.