Combine the brown sugar, cocoa powder, paprika, salt, onion powder, garlic powder, chili powder, and pepper in a small bowl.
Flip the ribs bone side up and insert a dinner knife just under the white membrane that covers the meat and bones. Gently peel the membrane off.
Coat the ribs on both sides generously with the mixture, reserving about 1/3 of a cup. Wrap the ribs in foil and refrigerate at least 4 hours or up to 24 hours. Remove the ribs from the refrigerator and bring to room temperature. Preheat the oven to 250ºF.
Place the ribs, still wrapped in foil, on a baking sheet and cook for 2½ - 3 hours or until the ribs are tender. Remove the ribs from the oven and increase the oven temperature to 450ºF.
Place a wire rack on the baking sheet, remove the ribs from the foil, and place on the wire rack curved side up. Sprinkle half of the reserved rub on the top of the ribs and cook for 10 minutes. Flip the ribs over, sprinkle the rest of the reserved rub on top, and cook for another 10 minutes.