Preheat oven to 350 F. Lightly spray 9x13 inch baking dish with non-stick cooking spray.
In a large bowl, whisk the cake mix, cold root beer, oil and eggs until smooth batter forms. Pour batter into the prepared baking dish and bake in the preheated oven for 30 to 35 minutes, or until toothpick comes out clean in the center. Let the cake cool before frosting.
While the cake is baking, whip together the frosting, root beer and vanilla extract with a fork. (I just carefully mixed it in the frosting container.) Place the prepared frosting into the fridge, while the cake cools.
Frost the cake, cut into serving pieces and top with optional candy toppings!