In a small saucepan combine the 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 to thicken, 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 into thoroughly combined, about 1/2 minute. 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).
Pour the chocolate batter into the prepared pie crust and refrigerate until firm and cool, at least 4 hours.
Yield: One 9-inch pie
CHOCOLATE COOKIE CRUST:
Preheat oven to 375ºF.
In the bowl of a food processor or blender, process the graham crackers and Oreos 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. Bake the crust for 15 minutes.
Remove the crust from the oven and let cool completely before filling.
Yield: One 9-inch crust