Preheat oven to 275 degrees.
Trim brisket, removing all hard fat. You can refer to instructional videos on YouTube for detailed trimming techniques or purchase pre-trimmed brisket.
Coat all sides of the brisket with mustard and BBQ rub mixture.
Place the seasoned brisket on a deep sheet pan fitted with roasting racks. Cover with foil and place in the preheated oven. Bake for approximately 1 hour per pound or until the brisket reaches an internal temperature of 195 degrees in the middle. Remove from the oven and let it rest, covered, for 30 minutes.
Slice the brisket against the grain to ensure tenderness.
For the potatoes, season the outside of each potato with olive oil, salt, pepper, and garlic powder. Wrap each potato individually in foil and place them in a 400-degree oven for about 1 hour or until they are soft when gently squeezed.
Once the potatoes are cooked, carefully unwrap them and make a slit down the middle. Fluff the insides with a fork and add butter, shredded cheese, chopped brisket, BBQ sauce, sour cream, and dried chives as desired.