In a small saucepan combine sugar, heavy cream, buttermilk, cornstarch and pinch of salt, and whisk until smooth. Place over medium-high heat, and bring to a boil, whisking from time to time for the sugar and cornstarch to dissolve and the mixture thickens, about 5 minutes. Continue cooking at a low boil for an additional 5 minutes, whisking constantly.
In a mixing bowl, beat the egg yolks lightly. Pour 1/2 cup of the hot mixture into the egg yolks and whisk thoroughly. Pour the egg yolk mixture into the saucepan and whisk over the heat until thoroughly combined and very thick, 1 to 2 minutes. Pour the mixture into a mixing bowl, and whisk in the chocolate, butter and vanilla. Continue whisking until thoroughly combined (mixture will be very thick). Cover the mixture with plastic wrap placed directly on the surface and refrigerate until cooled to room temperature, about 30 minutes.
Once cooled, fold the Cool Whip into the cool chocolate pudding mixture, then spoon the chocolate mixture into the prepared pie crust and refrigerate until firm and cool, at least 4 hours.
When ready to serve, top the pie with Whipped Cream and serve immediately.
by The Savvy Kitchen