Combine sugar, cornstarch, salt, and egg yolks in a heavy saucepan; stir well.
Gradually stir in milk; add chocolate.
Cook over medium heat, stirring constantly, until chocolate melts and mixture is thickened and bubbly.Remove from heat; stir in 1/4 cup amaretto and vanilla.
Pour into pastry shell.
Place a piece of plastic wrap over filling; chill at least 8 hours.
Combine whipping cream, 1 tablespoon amaretto, and powdered sugar; beat at low speed of an electric mixer until soft peaks form.
Spread whipped cream mixture over pie; garnish with shaved chocolate, if desired.