Preheat oven to 250°F.
Combine brown sugar, chili powder, smoked paprika, onion powder, garlic powder, salt and pepper in a small bowl. Place ribs on a sheet of foil large enough to wrap around the rack. Pat dry rub all over both sides of meat, leaving ribs meat side down. Tightly wrap up the foil to make a packet, then place on a metal rack set over a baking sheet.
Bake ribs for 3 hours. Remove from the oven and carefully open foil packet. Transfer ribs to a cutting board and brush all over with 2 tablespoons maple syrup. Pour accumulated juices into a small saucepan, discard foil and set aside.
Place a new, large sheet of foil on the rack and arrange half the butter slices on top. Put the ribs on top, meat side down. Place the remaining butter slices on the bone side of the ribs and seal the foil packet. Return the ribs to the oven and bake another 1½-2 hours, or until the ribs are fork tender.
Meanwhile, bring reserved juices and remaining 3 tablespoons of maple syrup to a simmer. Reduce by half, until thick and syrupy, about 10 minutes. Remove from heat and stir in apple cider vinegar. Set aside.
When ribs are done, transfer to a clean baking sheet meat side up. Preheat broiler. Brush surface with sauce and broil for a few minutes, watching closely, until caramelized.
Cut the ribs and serve warm.