In a heavy medium saucepan, combine half-and-half and 8 ounces of the chocolate. Cook and stir over medium heat for 5 to 10 minutes or until mixture comes to a full boil and thickens. Gradually stir about half of the hot mixture into beaten egg yolks; return yolk mixture to hot mixture in saucepan.
Cook and stir over low heat for 2 minutes more. Remove from heat; stir in vanilla. Pour into eight 4-ounce pot de crème cups or small dessert dishes. Cover and chill for 1 hour.
For ganache, in a small saucepan, heat whipping cream until simmering. Remove from heat; whisk in the remaining 4 ounces chocolate until smooth. If desired, whisk in orange liqueur.
Divide warm ganache evenly among pots de crème, spreading to cover tops. Cover and chill for 2 to 24 hours. If desired, serve topped with whipped cream and hazelnuts. Makes 8 servings.