Step 1

FOR THE FILLING: 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. Whisk in vanilla. Pour filling into cooled pie crust. Press lightly greased parchment paper against surface of filling and let cool completely, about 1 hour. Refrigerate until filling is firmly set, at least 2½ hours or up to 24 hours.

Step 2

FOR THE TOPPING: Using stand mixer fitted with whisk attachment, whip cream and sugar on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until stiff peaks form, 1 to 2 minutes. Spread whipped cream evenly over chilled pie and serve.