Preheat the oven to 180°C/Gas 4. Spray a 23cm pie plate with cooking spray.
In a food processor, crumb the digestive biscuits until finely ground. Add the butter and water and mix until the crumbs clump together. Press the crumb mixture into bottom of pie dish and about 1 cm up the sides.
Place in the oven to bake for 10 minutes, then remove and set aside to cool.
For the filling:
Put the gelatine in a small bowl, add the boiling water and stir until dissolved. Set aside.
In a medium saucepan mix the sugar, cocoa, cornflour 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 heat, whisking constantly, for about 10 minutes, until the mixture thickens and comes to a boil.
Remove the pan from the heat, add the chocolate and stir until melted. Stir in the vanilla and dissolved gelatine. Pour the mixture into the pastry case and leave to set for 3 hours in the refrigerator.
Whip the cream with the sugar and top each serving of pie with a spoonful of whipped cream.