Preheat the oven to 350?F.
Melt the bittersweet and unsweetened chocolates in the top of a double boiler, or in a metal bowl set over a pot of barely simmering water, stirring, until smooth. Remove from the heat.
Lightly beat the egg yolks in a small bowl. Combine the sugar, cornstarch, and salt in a medium saucepan over medium heat. Add the milk in a stream, whisking constantly. Bring the mixture to a boil, whisking constantly to keep lumps from forming. Remove from the heat.
Add 1 cup of the hot milk mixture to the yolks and whisk until smooth. Add the yolk mixture to the saucepan with the hot milk and bring to a simmer over medium-low heat. Simmer, whisking constantly, until thick and smooth, about 3 minutes. Remove from the heat and whisk in the melted chocolate, unsalted butter, and 2 teaspoons of the vanilla. Transfer to a large bowl. Cover with plastic wrap, pressing the wrap down onto the surface to prevent a skin from forming, and refrigerate until cool.
Meanwhile, combine the flour, salted butter, and walnuts in a medium bowl and stir. Using your fingers, press this crust into a 9 x 13-inch baking dish, covering the bottom and coming a little bit up the sides. Bake the crust until golden brown, about 45 minutes. Let cool.
Next, combine 2 cups cream, 1/4 cup confectioners' sugar, and 3/4 teaspoon vanilla in a large bowl. Whisk until stiff peaks form. Combine the cream cheese and the 3 cups sifted confectioners' sugar in a large bowl and cream them together using an electric mixer. Fold the whipped cream into the cream cheese mixture until just combined.
Spoon the chocolate pudding into the prepared dish over the walnut crust. Top with the cream cheese filling, cover, and refrigerate for at least 2 hours or overnight.
Just before serving, combine the remaining 2 cups cream, remaining 1/4 cup confectioners' sugar, and remaining 3/4 teaspoon vanilla in a large bowl. Whisk until soft peaks form. Garnish with the whipped cream before serving.
Makes 10 to 12 servings