In a small saucepan combine sugar, 3/4 cup 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.
Place 1 3/4 cups heavy cream in a chilled mixing bowl and add the confectioners' sugar. Beat until stiff peaks form. Gently fold about 1/4 of the whipped cream 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. Refrigerate the remaining whipped cream until you are ready to serve the pie.
When ready to serve, top the pie with the remaining sweetened whipped cream and serve immediately.
Chocolate Cookie Crust
Yield:1 (9-inch) crust
Preheat oven to 375 degrees F.
In the bowl of a food processor or blender, process the graham crackers and chocolate cookies to make crumbs.
In a bowl mix together the cookie crumbs, sugar and butter with your hands. Press the mixture evenly into a 9-inch pie pan. (It will seem like an excessive amount of crumbs, but will compress down quite a bit with pressure.) Cover the crust with plastic wrap and top with another 9-inch pan, pressing quite firmly to make a smooth surface. Remove the pan and the plastic wrap from the pressed crust prior to placing it into the preheated oven. Bake the crust for 15 minutes.
Remove the crust from the oven and let cool completely before filling.