Preheat oven to 375 degrees F. In the bowl of a food processor, process wafer and pecans until finely ground. Add melted butter and pulse until mixture is moistened. Press into the bottom and up the sides of a 9-inch pie plate. Chill in freezer for 10 minutes, or until mixture is firm. Bake for 8-10 minutes. Transfer to a wire rack and let cool completely.

In a medium saucepan combine sugar, cocoa, cornstarch, salt. Whisk in milk. Heat mixture over medium-high heat, stirring constantly, until thick and bubbling, about 7 minutes.

Whisk egg yolks in a medium bowl. Slowly whisk half of hot milk mixture into egg yolks until thoroughly combined. Pour mixture back into saucepan and cook over medium heat, stirring constantly, until mixture returns to a boil. Pour mixture through a fine sieve into a large bowl. Stir in vanilla and butter, 1 piece at a time, until completely smooth.

Pour mixture into cooled crust and smooth the top. Cover the surface with plastic and refrigerate for at least 4 hours or overnight.

With an electric mixer beat the cream, sugar, and vanilla until soft peaks form. Spread whipped cream over pie. Sprinkle with pecans and serve.