Beat egg yolks with sugar until thick. Add cream and half-n-half. Cook in a double boiler until thick. Stir while cooking but do not let it come to a boil. Remove from heat and let cool for a few minutes before adding rum and a dash of nutmeg.
Store in the refrigerator until ready to use.
Notes
Adapted from Helen Crobitt's Cookbook, circa 1957, recipe named Mildred's Eggnog Sauce.