Heat oven to 350°F.

In a food processor, process the 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.

Whisk together sugar, cornstarch, salt, and yolks in a 3-quart heavy saucepan until combined well, then 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.

Whip the heavy cream until soft peaks form. Pile it on top of the pie to serve.