Beat egg yolks in a large bowl until thick and pale. Gradually beat in sugar until blended.
Stir together milk and next 2 ingredients in a 4-qt. saucepan. Cook mixture over medium heat, stirring constantly, 8 to 10 minutes or just until mixture begins to bubble around edges of pan. (Do not boil.)
Gradually stir half of hot milk mixture into egg mixture. Stir egg mixture into remaining hot milk mixture in pan. Cook mixture over medium heat, stirring constantly, 7 to 8 minutes or until mixture thickens and coats the back of a spoon and a candy thermometer registers 160°. (Do not boil.)
Remove from heat, and stir in cream. Pour mixture through a fine wire-mesh strainer.
Let cool 1 hour. Stir in nutmeg, vanilla, and bourbon. Chill 1 hour. Garnish each serving, if desired.