Prepare the Filling: In a large mixing bowl, whisk together root beer, milk, and vanilla pudding mix until smooth and thickened, about 2 minutes. If using, stir in the root beer extract.
Fold in Whipped Cream: Gently fold in 2 cups of whipped cream until the mixture is uniform and fluffy.
Fill the Crust: Pour the filling into the pre-made graham cracker crust, spreading evenly.
Freeze: Freeze the pie for at least 4 hours, or until firm.
Serve: Before serving, let the pie sit at room temperature for 5-10 minutes to soften slightly. Top with additional whipped cream and garnish with maraschino cherries.