Prepare crust first, let cool completely while preparing filling.
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 30 seconds. 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.
Notes
-I made the cookie crust as directed, but did not remove the white filling from the Oreos. Next time, I will half the cookie crust recipe. There were way too many crumbs for the crust. Even after pressing down, I had more than enough for another whole 9 inch tin.
-Make sure to measure your tin, anything larger than a 9 inch tin will be too big for the amount of filling.
-I used Baker's Semisweet chocolate.
-A few of the reviews on FN were written by peeps who used store bought crust.
My 2 cents: The crust if half of what sets this recipe apart. Take the extra 3 minutes, and make your own. Well worth it.