Follow the steps in order for the best result.
In a large saucepan set over medium heat, reduce the root beer to approximately 1 cup. This reduction process will take about an hour.
Once the root beer is reduced, add the apple cider vinegar, ketchup, yellow mustard, Worcestershire sauce, Tabasco sauce, salt, and pepper to the saucepan.
Stir all ingredients together thoroughly and allow the sauce to simmer for 20 minutes.
To finish the sauce, whisk in the cold butter. This will add extra body and enhance the overall flavor.