For the crust: Combine the sugar, graham cracker crumbs, cocoa and melted butter and mix well. Press the mixture into the bottom and up the sides of a 9-inch pie pan. Refrigerate.
For the filling: Melt the chocolate using a double boiler or a stainless steel bowl placed over a kettle of boiling water. Dissolve the gelatin in cold water. Cream together the sugar, egg yolks and vanilla. Heat the milk in a medium saucepan until hot but not boiling. Slowly pour the milk over the egg mixture in a thin stream, stirring as you go so the eggs don't scramble. Pour the mixture back into the saucepan. Heat over low, stirring with a wooden spoon, until the mixture coats the back of the spoon. Do not boil. This will take up to 15 minutes, be patient. Stir in the gelatin/water mix and melted chocolate. Stir until smooth. Strain into a bowl and let cool. If you don't mind a few flecks you don't need to strain it.
Whip the heavy cream to a medium peak. Fold 2/3 into the cooled chocolate custard. Save the remaining 1/3 for garnish. After folding carefully for a full 5 minutes I still had a few flecks of cream in the finished pie. Pour the custard into the prepared pie crust. Refrigerate for 2 hours. Decorate with whipped cream and chocolate shavings before serving