1.   Add cream and cook over high heat until thick enough to coat a spoon.

2.   Cook milk, eggs, nutmeg and sugar over low heat until mixture thickens slightly and coats a spoon.

3.   Cook over medium-low heat, stirring constantly with a wooden spoon until custard begins to thicken and coat the spoon.

4.   Cook the mixture over medium-low heat, stirring until the butter is melted and custard is thick enough to coat a spoon.

5.   Cook, stirring with a wooden spoon, until jelly is melted and bubbling and coats spoon lightly.

6.   If sauce seems too thin, add a little of the arrowroot mixture and simmer until it lightly coats a spoon.

7.   If you can still see the path of your finger after a few seconds, the spoon is coated.

8.   Return the mixture to the saucepan and cook over moderately low heat, stirring with a wooden spoon, until the sauce visibly thickens and coats the spoon.

9.   Sauce should thicken enough to lightly coat a spoon.

10.   When mixture thickens enough to coat a spoon, remove from heat and stir in rum.