Line chilled pie shell with double layer of aluminum foil, covering edges to prevent burning, and fill with dry beans or pie weights. Bake on foil-lined rimmed baking sheet until edges are set and just beginning to turn golden, 25 to 30 minutes, rotating sheet halfway through baking. Remove foil and beans/weights, rotate sheet, and continue to bake crust until golden brown and crisp, 10 to 15 minutes. Transfer sheet to wire rack and let cool completely, about 30 minutes.
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 2 1/2 hours or up to 24 hours. Spread whipped cream attractively over chilled pie and serve.
Nutrition information per each of 10 servings:
Calories375Fat25 gSodium190 mgCarbohydrates32 gSaturated fat14 gAdded sugars12 gProtein6 gCholesterol44 mgDietary fiber3 g
Exchanges per serving: 2 carb, 1 medium-fat protein, 4 fat.