Heat oven to 350 degrees.
For crust, in a food processor, process hazelnuts, sugar and cocoa powder until fine. Place in a bowl and add melted butter. Press onto bottom and up side of a 9-inch pie plate. Bake until slightly crisp, about 25 minutes, and cool on a rack.
Meanwhile, make the filling. In a 3-quart heavy saucepan, whisk together sugar, cornstarch, salt and yolks until combined well. Add milk in a stream, whisking. Bring to a boil over moderate heat, whisking, then reduce heat and simmer, whisking, 1 minute (filling will be thick).
Force filling through a fine-mesh sieve into a bowl, then whisk in chocolate, butter and vanilla. Cover surface of filling with a buttered round of wax paper and cool completely, about 2 hours.
Spoon filling into crust and chill pie, loosely covered, at least 6 hours before serving.
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