Boiled Custard has been a Perry family Christmas tradition since before I was born, and I keep the tradition alive by making it with my nephew each year.
Combine milks and sugar in heavy pot over double boiler. Stir constantly until this almost boils. (This can take up to 30 minutes.) Slowly and rapidly whisk in the eggs to keep them from “cooking” as added. Bring mixture to a gentle boil. Reduce heat to medium. Stir often and cook for about 45 minutes or until custard coats metal spoon.
Remove from heat. Add 1 tablespoon vanilla and more sugar if necessary. Cool in freezer until freezing begins around edges. Refrigerate up to 2 weeks. Consistency should be somewhat thick but pourable.
*Back in the day, “sweet” (whole) milk would be written to differentiate from (cultured) buttermilk or sour cream.