Preheat smoker or grill to 225-250°F (107-121°C). If using a conventional oven, preheat it to the lowest setting.
Mix brown sugar and ketchup in a bowl until smooth. Season ribs with garlic powder, onion powder, dry mustard, black pepper, salt, and half of your barbecue sauce mixture for basting/marinating overnight or at least 2 hours.
Wrap the seasoned rib slabs tightly in foil. Cook directly on smoker grates until internal temperature reaches about 195°F (90°C), approximately 3-4 hours, flipping halfway through cooking time.
Unwrap and brush with remaining barbecue sauce for the last 20 minutes of smoke or bake in a preheated conventional oven at about 250°F (121°C) until internal temperature reaches your desired tenderness, approximately another hour.
Let ribs rest off-heat for at least 15 minutes before slicing against the grain and serving.