Remove the corned beef brisket from its package and place it in a container. Cover with cold water and soak overnight in the refrigerator. Reserve the seasoning packet.
In a small bowl, combine the salt, black pepper, garlic powder, mustard powder and paprika. Mix in the seasoning packet from the corned beef.
When you are ready to cook, remove the corned beef from the water and pat dry.
Rub the corned beef down with a layer of yellow mustard.
Apply the seasoning rub to both sides of the corned beef. Depending on the size of your corned beef and size of the seasoning packet you may not need to use all of the seasoning. See note 1.
Heat your smoker to 250 degrees. Place the seasoned corned beef on a wire rack above a foil pan. Place about 2 cups of beef broth in the foil pan. Smoke the corned beef until the bark appears set, typically about 180-185 degrees internal temp.
At this point remove the corned beef from the wire rack and place into the foil pan. Cover with foil and return to the smoker. Cook to about 200 degrees internal temperature.
Remove the corned beef to a cutting board and cut it into cubes. Increase the temperature of your smoker to 300 degrees.
In a small bowl combine the Guinness with the BBQ sauce. Strain the liquid from the foil pan into a measuring cup. Add about ½ of a cup of the au jus to the sauce. Stir well to combine.
Toss the cubed smoked corned beef in some of the sauce in a new foil pan. Use just enough to coat all of the pieces well. Reserve the rest of the sauce.
Return the sauced corned beef to the smoker. Cook, uncovered, at 300 degrees for about 30 minutes or until the sauce is set.
Serve the smoked corned beef burnt ends warm with the reserved BBQ sauce.