Heat milk in a heavy-bottomed saucepan over medium heat until just under a boil. Remove from heat. Whisk together egg yolks and next 3 ingredients in a stainless steel bowl. Whisk a small amount of hot milk into yolk mixture to temper. Whisk yolk mixture into remaining hot milk in saucepan.
Cook over medium-low heat, whisking slowly, 7 minutes or until mixture thickens and starts to bubble. Remove from heat, and stir in chopped chocolate, softened butter, and, if desired, bourbon.
Place plastic wrap on surface to prevent skin formation. Chill over an ice bath until completely cool. Filling may be chilled up to 2 days before using.
*To bring eggs to room temperature safely, remove from refrigerator, and place in a bowl for 30 minutes.