Make the eggnog base. In a large cool pot, whisk together the egg yolks and sweetened condensed milk.
Simmer. Once well combined, add the whole milk, vanilla, cinnamon, nutmeg, rum (if you want) and stir until well combined. Cook over low heat and continue to a simmer and stir occasionally for 10 minutes or until the egg nog is thick enough to coat the back of a spoon. (Note: Don’t boil or the eggs and milk will curdle and get chunky.)
Strain. Put the eggnog through a fine mesh strainer and store in the refrigerator, covered with plastic wrap for at least 2 hours or until ready to serve.
Notes
You can store your leftover eggnog in the refrigerator for 2-4 days.