Bake and cool pie crust according to package directions.
Whisk milk, syrup and yolks in a large bowl to blend.
Whisk flour and cornstarch in medium bowl.
Gradually whisk 1 cup milk mixture into flour mixture. Gradually whisk in remaining milk mixture.
Transfer to a large saucepan. Whisk over medium heat until custard thickens and boils, about 8 minutes. Whisk in 2 teaspoons vanilla. Pour into prepared pie crust. Chill until cold, about 3 hours. (You can make up to this point a day ahead--just cover and keep chilled.).
Beat cream, sugar and remaining 1/2 teaspoon vanilla in medium bowl until peaks form. Spread whipped cream over pie adn sprinkle with walnuts.