Preheat the oven to 350 degrees F. Break the graham crackers into small pieces and place in a food processor. Pulse to blend into fine crumbs. Add the butter, cocoa powder and salt and pulse, in 2 second intervals, until combined and the consistency of wet sand. Press the mixture into and up the sides of a 9-inch pie plate. Bake until toasted and fragrant, about 8 minutes. Cool before filling, about 15 minutes.
Scoop the dulce de leche into a medium bowl. Using a rubber spatula, fold in the walnuts, chocolate chunks and orange zest. Pour the filling into the prepared pie shell and refrigerate until chilled all the way through and set, about 3 hours.
Just before serving, use a hand-mixer to whip the heavy cream and vanilla in a medium until soft peaks form. Sprinkle the flake salt on the pie, top with the whipped cream and grate the unsweetened chocolate over the top. Serve.
Recipe courtesy of Giada De Laurentiis