In a medium bowl, beat egg yolks and sugar together until smooth. Set aside.
Heat milk and cream until mixture just starts to bubble around the edges.
Stream milk mixture into egg mixture, whisking constantly.
Return to saucepan and cook over medium low heat until mixture thickens just enough to lightly coat a spoon, about 5 minutes.
Remove from heat.
Stir in nutmeg, vanilla, and bourbon (if using).
Cover and refrigerate until cold.
Pour into ice cream freezer and freeze according to manufacturer's instructions.
Scoop into covered container and freeze until solid.