1. Begin reducing the root beer by placing it in a pot along with the sugar, blacken, and vanilla. Cook at a high temperature, and let it boil away until it's reduced to a syrup. set this aside.
2. Prepare the roast by cutting diagonal slits in the fat. This will let the glaze get into the meat, and not just stay on top. coat liberally with the salt, pepper, and blacken. Add enough oil to make a shiny mixture, and rub it all into the meat.
3. Place the meat on a rack in a pan and set it into the oven. 20 minutes later, turn the oven down to 300- degrees without opening the door.
4. 20 minutes after you've turned the oven down, glaze the top of the roast with the syrup. It helps to do this fairly quickly because the meat isn't cooking during this time. You can baste the meat a couple of times while it's cooking.
5. Thin any left over syrup with a dark colored fruit juice. I usually have Cran-Ras in the frig. for cocktails, so I used that. Pour this into the bottom of the pan. check during cooking to make sure that the liquid hasn't dried up, and add more juice or beef broth if necessary.
6. Cook until the internal temperature reaches 145 degrees. I use an electronic meat thermometer that i can actually wander up to 7 boats away, and still have the signal.
Total cooking time is about 1 1/2 hours, depending on the thickness of the roast, and your oven
7. After the meat has reached 145 degrees. Remove it from the oven, place it on a lipped platter, and cover with aluminum foil. If you leave your meat thermometer in at this point, you'll notice that the temperature will rise about 10 degrees. Let it rest, covered, for at least 20 minutes. This should provide enough time to make the gravy (sauce) that you'll serve with the meat.
8. Take the rack out of the pan, and set the pan on top of a burner over high heat, and add 32 ounces of beef broth. Let this come to a boil, then scrape up any brown bits (fond) that have stuck to the bottom of the pan. Thicken with a corn starch slurry. You make a "slurry" by placing 2 tablespoons of cornstarch in a cup, then add a little bit of cold water to make a paste. Add more water until it becomes pourable. Add this to the boiling sauce, while stirring constantly. Let the sauce come back to a boil, and add more cornstarch slurry as needed.
Note: This makes a very rich, sauce that's a little on the sweet side with a mild kick from the blacken. Use it sparingly.