In a bowl, whisk the egg yolks until they lighten slightly in color. While continuing to whisk, slowly add the sugar and continue to beat until it is completely dissolved. Set aside.
In a medium saucepan over high heat, combine the milk, heavy cream, and nutmeg. Bring the mixture to a boil, stirring occasionally. Remove from the heat and very slowly add this hot mixture to the egg and sugar mixture while continuing to whisk. Return everything to the saucepan and cook until the mixture reaches 160 degrees F. Remove from heat, stir in liquor (optional), pour into a bowl, and set in the refrigerator to chill.