Preheat your oven to what is suggested on the box (for me that was 350).
Mix your cake mix according to the instructions, adding in 1 1/2 tsp root beer concentrate
Bake the cake according to the instructions (for me that was 28 minutes). Let the cake rest for about 10 minutes after you take it out of the oven. Then poke holes throughout the cake using the bottom of a wooden spoon.
Pour the sweetened condensed milk over the cake, filling all of the holes as evenly as possible.
Cover and refrigerate the cake overnight. Serve with vanilla ice cream.