Whisk sugar, cornstarch, cocoa and salt together in large saucepan. Whisk in milk until incorporated, making sure to scrape corners of saucepan. Place saucepan over medium heat, cook whisking constantly, until mixture is thickened and bubbling over entire surface, 8 to 10 minutes. Cook 30 seconds longer, remove from heat. Add chocolate and butter and whisk until melted and fully incorporated. Pour filling into the cooled pie crust. Press lightly greased parchment paper against surface of filling

Let cool completely, about 1 hour. Refrigerate until filling is firmly set, at least 2 1/2 hours or up to 24 hours.

Spread whip cream on top if desired.