Prepare Pudding: Whisk together first 4 ingredients in a large saucepan. Slowly whisk in 1 cup buttermilk to make a smooth paste. Whisk together 1 1/4 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 semisweet 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 to 24 hours. Prepare Buttermilk Cream: Beat 3/4 cup cream, 1/4 cup powdered sugar, and 1/4 cup buttermilk at high speed with an electric mixer until soft peaks form. (Do not overbeat.) Divide pudding among 6 (6- to 8-oz.) bowls; dollop with buttermilk cream.
Delicious, but should be served only in small amounts, as it is very rich and dense.