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.