Set aside and refrigerate 1/2 cup whipped topping for garnish.
In a large bowl, whisk the root beer, milk, root beer flavoring and pudding mix for 2 minutes.
Fold in half the remaining whipped topping.
Spread into graham cracker crust.
Spread remaining whipped topping over pie. Freeze for at least 8 hours or overnight.
Dollop reserved whipped topping over each serving; top with a maraschino cherry if desired.