Add the sugar, cocoa, cornstarch, and salt to a medium saucepan and whisk to combine.

Pour the milk in slowly, whisking as you pour it in. Keep whisking to get the lumps out.

Cook on medium heat, whisking often, until it boils. 

Boil for about two minutes while you whisk. 

Remove from heat, add the butter and vanilla extract and whisk.

Pour into the graham cracker crust and let it set up overnight in the refrigerator. 

The next day, top with homemade whipped cream.