Preheat oven to 275°F (135°C). Pat ribs dry with paper towels.
Flip ribs bone-side up and remove the membrane for better tenderness.
Mix brown sugar, paprika, salt, pepper, garlic powder, onion powder and cayenne in a bowl.
Brush both sides of ribs with Dijon mustard or a little oil to help the rub adhere.
Generously rub the spice mixture over both sides of each rack, massaging into the meat.
Wrap each rack tightly in aluminum foil, bone-side down; add a tablespoon of water or apple juice inside each packet if desired.
Place wrapped racks on a baking sheet and bake 2 to 2½ hours until meat is tender and pulls back from the bones about ¼–½ inch.
Carefully open foil, drain excess liquid and transfer ribs to a foil-lined tray, bone-side down.
Brush a generous layer of barbecue sauce over the ribs.
Increase oven to 425°F (220°C) or broil briefly for 6–10 minutes until the sauce is bubbling and caramelized; watch closely to avoid burning.
Let ribs rest 5–10 minutes, then slice between the bones and serve.