Combine butter and chopped chocolate in a large glass or metal bowl; set aside

In a large saucepan, whisk together sugar, cornstarch, cocoa, and salt. Slowly whisk in the 1/2 cup of cream till mixture is completely smooth. Whisk in the egg yolks.

Add the milk and whisk till mixture is well blended. Turn heat to medium, cook and stir till mixture comes to a boil. Boil for one minute, stirring constantly. 

Place a strainer over the bowl of chocolate and butter. Pour hot chocolate mixture through the strainer to remove any lumps. Let sit for a couple of minutes, then stir till butter and chocolate are melted.

Place bowl of pudding in a sink of ice water, stir gently for a few minutes till cooled to room temperature. 

Pour pudding into pie shell. Cover with plastic wrap and chill for several hours or overnight.

Just before serving, beat the 1 cup of cream till soft peaks form. Beat in the powdered sugar and vanilla. Spread over pie and garnish with chocolate curls if desired.