BEAT eggs, sugar and salt in large heavy saucepan until blended. STIR IN 2 cups milk.
COOK over low heat, stirring constantly but gently, until mixture is just thick enough to just coat a metal spoon with a thin film and temperature reaches 160°F, about 15 minutes. Do not allow to boil. REMOVE from heat immediately.
STIR IN remaining 2 cups milk and vanilla. REFRIGERATE, covered, until thoroughly chilled, several hours or overnight.
Just before serving, stir brandy, liqueur, rum or bourbon into eggnog, if desired. For a festive presentation, garnish with whipped cream, ground nutmeg, cinnamon sticks or candy canes.
Secrets of success: Low heat, a heavy sauce pan, constant stirring and patience are the keys to making the eggnog. If you increase the cooking temperature to try to speed the process along, the mixture is likely to curdle. Stirring constantly, making sure to cover the entire bottom and corners of the pan, prevents scorching and ensures that the mixture heats.
Watch carefully and test frequently toward the end of the cooking time, after about 10 to 12 minutes. The last few minutes are crucial. Undercooked eggnog will be thin and watery; overcooked custard will curdle. The difference is a matter of only a few degrees.
For perfectly smooth eggnog: Pour through a sieve before chilling.
For a richer eggnog: Substitute half-and-half or light cream for some of the milk.
To keep eggnog cold during a party, set punch bowl or pitcher in a bed of crushed ice, or freeze some of the eggnog in ice cube trays or ice ring using a bundt pan and add to bowl right before party.
Use leftover eggnog in French toast or pancake batter.
This recipe is a good source of protein.