Preheat oven to 450.
Press pie crust into a 9" pie pan and bake on lower rack for 10-12 minutes (until crust is browned).
Allow to cool completely.
Combine sugar, cocoa, cornstarch, and salt in a medium pan.
While stirring constantly, slowly add in milk and bring to a boil over medium heat.
Remove from the heat and stir in vanilla.
Pour into pie crust and chill until firm.
Whip heavy cream at high speed.
Slowly add 3 tablespoons sugar.
Beat until stiff peaks form.
Spread whipped cream over pie.
Sprinkle with shaved chocolate (optional).
Refrigerate until ready to serve.