In a medium saucepan, whisk together the sugar, cocoa powder, cornstarch, and salt.

Slowly whisk in the milk until smooth.

Cook over medium heat, whisking often, until the mixture thickens and begins to bubble.

Continue cooking for 1 minute while whisking constantly.

Remove from the heat and stir in the chopped chocolate, butter, and vanilla until smooth.

Pour the chocolate filling into the cooled pie crust and smooth the top.

Press plastic wrap directly onto the surface of the filling if desired.

Refrigerate for at least 4 hours, or until fully set.

Spread whipped topping over the chilled pie.

Garnish with chocolate shavings, mini chocolate chips, or cocoa powder if desired.

Slice and serve cold.