Roll the pie dough out on a lightly floured board to about 1/4-inch thick, then fit into a 9-inch pie dish and crimp the edges.
Place a piece of parchment paper into the pie crust and fill with dried beans or pie weights. Bake until the crust is starting to set and the edges are browning, 20 minutes. Remove the parchment and beans, and continue to bake until completely golden and crisp looking, another 5 minutes. If the edges start browning before the center cooks, cover them with some foil. Let cool completely.
Melt the bittersweet chocolate over a double boiler. Using a pastry brush, paint the inside of the pie shell with the melted chocolate in a thin even layer. Set aside to harden.
For the chocolate filling: Meanwhile, in the top of a double boiler heat 2 cups of the milk until almost, but not quite, boiling. Add the chocolate and stir until it's completely melted. Set aside off the heat.
In a big bowl, whisk together the sugar, cornstarch, flour and salt. Whisk in the remaining 1/2 cup milk.
Stir the flour mixture into the hot chocolate mixture. Return the double boiler to the heat, whisk in the egg yolks, and cook, whisking constantly, until the mixture is very thick, about 20 minutes. Remove from the heat, then transfer to another bowl. Cover with plastic wrap directly touching the surface, and refrigerate until cold.
At least 3 hours before you are ready to serve, add the chocolate filling to the pie shell, and refrigerate.
For the whipped cream: Right before serving, cream together the mascarpone with the sugar and vanilla bean seeds in a stand mixer fitted with the whisk attachment. Add the heavy cream and continue to beat until it holds medium-stiff peaks.
Add the whipped cream to a pastry bag fitted with a large star tip. Pipe the cream generously on top of the pie. Sprinkle with chocolate curls and shavings, and serve.