In a medium saucepan, mix together cocoa powder, sugar, cornstarch, salt and milk. Whisk until smooth.
Cook over Medium-High Heat, stirring often until milk starts to heat. Once the milk starts to warm, stir constantly, making sure to scrape sides and bottom of pan.
Bring to a boil, stirring constantly
Let mixture actively boil for 1 minute, continuing to stir.
Remove from heat and stir in butter and vanilla. Whisk to blend
Pour in to cooled and baked 9" pie crust
Cover with plastic wrap, making sure that entire surface of the filling is covered (this will prevent the filling developing a "skin" as it cools)
Chill in the refrigerator 4 - 6 hours or until firm.
Top with whipped cream and chocolate shavings or mini chocolate chips