Preheat the oven to 350 degrees F. Spray a 9-inch pie plate with cooking spray.
In a food processor, process the graham crackers until finely ground. Add butter and water and process until the crumbs clump together. Press the crumb mixture into bottom of pie plate and about 1-inch up the sides. Bake for 10 minutes, then let cool.
In the meantime, make the pudding. Put the gelatin in a small bowl, add the boiling water and stir until dissolved. Set aside.
In a medium saucean mix the 2/3 cup of sugar, cocoa, cornstarch and salt. Gradually add half of the milk, whisking until smooth. Whisk in the rest of the milk. Turn the heat on to medium and cook, whisking constantly, until the mixture thickens and comes to a boil, about 10 minutes.
Remove from the heat. Add the chocolate and stir until melted. Stir in the vanilla and reserved gelatin. Pour the mixture into the pie crust and let set for 3 hours in the refrigerator.
Whip the cream and remaining 1/2 teaspoon of sugar. Before serving, garnish the pie with the whipped cream.