Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch (or two 8-inch round) cake pan(s).
Mix the cake batter:
In a large bowl, combine the cake mix, root beer, vegetable oil, eggs, and vanilla extract. Beat on low speed for 30 seconds, then on medium speed for 2 minutes until the batter is smooth.
Bake:
Pour the batter into the prepared pan(s) and bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean.
Cool:
Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
For the Frosting:
Make the frosting:
In a medium bowl, beat the softened butter until creamy.
Gradually add the powdered sugar, alternating with root beer, until the frosting is smooth and spreadable.
Add root beer extract, if using, for an extra burst of flavor.
Frost the cake:
Spread the frosting evenly over the cooled cake.
For Topping (Optional):
Garnish the cake with dollops of whipped cream or piped vanilla frosting.
Top with maraschino cherries and a sprinkle of chocolate shavings or sprinkles for a fun finish.