Step 1: In a medium bowl, whisk together the egg yolks and 1/4 cup of the granulated sugar until light yellow and creamy.
Step 2: In a saucepan over medium heat, combine the whole milk, heavy cream, the remaining 1/4 cup sugar, vanilla extract, nutmeg, and salt. Heat until steam rises and small bubbles form around the edges, but do not boil.
Step 3: Slowly temper the egg mixture: While continuously whisking the egg yolks, gradually pour about half of the hot milk mixture into the egg yolks. This slowly raises the temperature of the eggs without scrambling them.
Step 4: Pour the tempered egg mixture back into the saucepan with the remaining hot milk mixture. Return to medium-low heat.
Step 5: Cook, stirring constantly with a wooden spoon or spatula, until the mixture thickens enough to coat the back of the spoon (reaching approximately 160°F or 71°C). Do not boil.
Step 6: Remove the saucepan from the heat. Pour the eggnog through a fine-mesh sieve into a clean bowl or pitcher to remove any potential bits of cooked egg or spice. Stir in the optional alcohol, if using.
Step 7: Cover the eggnog and refrigerate for at least 4 hours, or preferably overnight, to allow the flavors to meld and for it to chill thoroughly.
Step 8: When ready to serve, whisk the chilled eggnog briefly to ensure it's smooth. Pour into glasses and garnish generously with additional freshly grated nutmeg.