Prepare Pudding by whisking together first 4 ingredients in a large saucepan.
Slowly whisk in 1 cup buttermilk to make a smooth paste.
Whisk together 1 ¼ cups cream, egg yolks, and egg in a 2 cup glass measuring cup.
Slowly add cream mixture to buttermilk mixture, whisking constantly until well blended.
Cook pudding over medium heat, whisking constantly, about 10 minutes or until mixture begins to boil and thicken. Remove from heat.
Add chopped semi-sweet chocolate, whisk until smooth.
Add butter and vanilla, and whisk until butter melts and pudding is smooth.
Transfer to a bowl and place plastic wrap directly on warm pudding to prevent a film from forming. Chill 4-24 hours.