Combine sugar, cornstarch, gelatin, and salt in a medium saucepan. Stir in milk. Cook over medium heat, stirring constantly, until thickened, 7 to 10 minutes. Reduce heat to low.

Beat eggs yolks and vanilla extract in a bowl. Slowly whisk in a small amount of the hot milk mixture. Slowly pour mixture back into the pan, whisking constantly. Cook and stir over low heat until custard is very thick, about 2 minutes. Do not let boil. Remove from heat.

Stir in chocolate until melted. Pour into the prepared graham cracker crust and chill for 2 hours.

Combine heavy cream, sugar, and vanilla extract in a bowl. Beat using an electric mixer on medium speed until soft peaks form. Pipe onto the chilled pie.