Heat the milk, cream and nutmeg in a saucepan over medium heat, stirring occasionally, until steaming, 5 to 7 minutes. Reduce the heat to low and keep warm.
Whisk the egg yolks and sugar in a separate saucepan until smooth. Cook over low heat, whisking constantly, until the mixture coats a spoon and registers 160 degrees F on a thermometer, about 25 minutes. Remove from the heat and gradually whisk in the warm milk mixture. Let cool 30 minutes.
Transfer the eggnog to a pitcher, cover and refrigerate until cold, at least 3 hours and up to 1 day. Pour 1 to 2 tablespoons bourbon into each glass. Top with the eggnog and sprinkle with nutmeg.