To make the crust, preheat the oven to 350 degrees F. Crumble the cookies in the work bowl of a food processor and process until finely ground. Alternatively, you can put them in a bag and whack them with a rolling pin until finely crushed. Pour the butter into the crumbs and mix (hands are the best for this) until the butter is fully incorporated and the texture is that of wet sand. Firmly press the crumbs against the sides of a 9-inch (23-cm) pie pan, then against the bottom of the pan (the underside of a measuring cup works well for smother the bottom crust). Chill the crust for at least 15 minutes to help prevent it from crumbling when serving.
Bake the crust for 10 minutes, and then let it cool completely.
Make the filling: Put the egg whites in the fridge to keep cold. Beat the egg yolks with 1/3 cup (65g) of the sugar until light and frothy. Beat in the orange juice, liqueur, vanilla, and salt. In a separate small bowl, sprinkle the gelatin over 1/3 cup (75ml) cold water to allow it to bloom.
In a double boiler or in a metal bowl set over a pot of simmering water, whisk the yolk mixture constantly until it's slightly thickened and just coats the back of a spoon (a digital-read thermometer will read 175 degrees F when it's done). Remove it from the heat and slightly whisk in the gelatin mixture until it is completely dissolved.
Put the mixture over an ice bath or in the refrigerator, stirring occasionally with a spatula, until it reaches room temperature.
Once the yolk mixture is at room temperature, make a meringue. In a clean, cold metal bowl. beat the whites until soft peaks form. Slowly add the remaining 1/3 cup (65g) sugar and beat until stiff peaks form.
Gently fold the meringue into the yolk mixture in 3 to 4 batches, until it is fully incorporated. Be careful not to over do it, so the chiffon doesn't deflate. Pour the chiffon into the pie shell and refrigerate it for at least 2 hours.
Make the topping: Using a hand mixer, or by hand with a whisk, whip the cream, sugar, liqueur and vanilla together until stiff peaks form. Dollop the whipped cream onto the pie surface. Sprinkle with orange zest and serve. This pie can be made ahead without the topping and refrigerated for up to 1 week, covered in plastic wrap. Make the topping just before serving.