Rompope (Mexican Eggnog)

In a large saucepan, combine 4 cups of whole milk, 1 cinnamon stick, 3 whole cloves, and 1 teaspoon of vanilla extract. Heat over medium heat until the milk is steaming but not boiling, stirring often.
Add 1 cup of granulated sugar to the milk. Stir until the sugar is fully dissolved.
Reduce the heat to low and let the milk simmer gently for about 10 minutes to infuse the flavors. Stir occasionally.
Remove the saucepan from the heat and let the spiced milk cool for about 15 minutes.
In a large bowl, whisk 6 large egg yolks until they are pale yellow and slightly thickened.
Slowly pour about 1 cup of the warm (not hot) spiced milk into the egg yolks while whisking constantly to prevent curdling.
Pour the egg yolk mixture back into the saucepan with the remaining spiced milk, stirring constantly.
Return the saucepan to low heat. Cook, stirring constantly, until the mixture thickens slightly and coats the back of a spoon. Do not let it boil or it may split.
Stir in 1/4 teaspoon of ground nutmeg.
Remove from heat and strain the mixture through a fine mesh sieve to remove the cinnamon stick and cloves.
If using, stir in 1/2 cup of rum or brandy.
Let the rompope cool to room temperature, then refrigerate until chilled. Serve cold.