Heat the Milk and Cream: In a small saucepan (1-quart), combine the milk and cream over medium heat. Let it warm gently while you prepare the other ingredients.

Mix the Egg and Sugar: In a small bowl, whisk together the egg yolk, sugar, and nutmeg until smooth and well combined.

Temper the Egg Mixture: Once the milk mixture begins to simmer, remove it from the heat. Slowly whisk 1 tablespoon of the warm milk into the egg mixture. Repeat this step, adding one tablespoon at a time and whisking thoroughly, until you've incorporated about half of the milk mixture into the eggs.Tip: This tempering process gradually warms the egg to prevent it from curdling when added to the hot milk.

Combine and Heat: Pour the tempered egg mixture back into the saucepan with the remaining milk and cream. Return the pan to the stove over low heat.

Cook the Eggnog: Stir or whisk the mixture constantly for about 10 minutes, or until it reaches 160°F on a candy thermometer. If you don’t have a thermometer, cook until the eggnog thickens slightly and coats the back of a spoon. Avoid letting it boil, as this can cause curdling.

Add the Vanilla: Remove the pan from the heat and stir in the vanilla extract.

Serve or Chill: Pour the eggnog into a mug. If desired, add whiskey, rum, bourbon, or brandy to taste. For best flavor, refrigerate for at least 30 minutes before enjoying.