Large bowl whisk together cocoa, flour and sugar. Slowly whisk in 1/4 cup of milk to form a paste. Slowly add remaining milk. Transfer to sauce pan and bring to boil over medium heat, stirring constantly with whisk. Remove and add chopped chocolate and vanilla. Stir until melted and mixture is smooth. Pour in pie crust. Cover with plastic and refrigerate until cold and set.
Whip heavy cream with mixer until soft peaks form. Add sugar and vanilla and whip until stiff. Remove plastic wrap from pie and swirl cream on top. Sprinkle with cocoa and serve.
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