Preheat the oven to 375°. In a medium bowl, mix all the crust ingredients until the dough resembles wet sand. Press evenly onto the bottom of an 8-by-8-inch baking dish. Bake until the top is evenly golden brown, 20 to 25 minutes. Let cool completely.
Meanwhile, in a large saucepan, bring the buttermilk, milk, granulated sugar, cornstarch and salt to a boil over medium-high, stirring until the sugar dissolves. Reduce the heat to medium-low and simmer, stirring often, until the pudding thickens slightly, about 8 minutes.
In a medium bowl, whisk the egg yolks just to blend. Whisk in 1/2 cup pudding; continue adding the pudding by the 1/2 cupful, whisking to blend between additions. Return to the saucepan. Cook over medium-low heat, stirring constantly, until the pudding coats the back of a spoon, about 3 minutes. Remove from the heat and transfer to another medium bowl. Add the chopped chocolate, butter and vanilla; stir until the chocolate melts and the mixture is smooth. Cover by placing a sheet of plastic wrap directly onto the mixture. Refrigerate until completely cool, about 30 minutes.
While the pudding cools, in a large bowl, using an electric mixer, beat the cream and confectioners’ sugar on high speed until soft peaks form. Fold 1/2 cup whipped cream into the chocolate pudding; spread over the cooled crust in an even layer. Spread the remaining whipped cream on top. Refrigerate at least 4 hours and up to 8 hours. Just before serving, use a vegetable peeler to shave some of the dark chocolate over the pie. Cut into 6 pieces and serve chilled.
Note: Going gluten-free? No problem! Just substitute any non-wheat flour for the regular flour used in the crust.