You can make this slightly sweet sauce up to one day ahead and reheat it over low heat. Do not boil.
Combine egg yolks and syrup in top of a double boiler, stirring well with a wire whisk. Gradually stir in half-and-half. Bring water to a boil. Reduce heat to low; cook, stirring constantly, until mixture thickens and coats back of a spoon. Remove from heat, and stir in vanilla. Serve warm or chilled.