To make the Gochujang barbecue sauce: Combine ketchup, gochujang, hoisin, oyster sauce, soy sauce, brown sugar, fish sauce, plum extract, onion and garlic powder, black pepper, ground ginger, mustard paste, and chipotles in a medium saucepan. Stir and bring to a simmer over medium-low heat. Cook 8–10 minutes, stirring occasionally. Remove from heat and allow to cool. Carefully transfer mixture to a blender, and add apple cider vinegar. Cover, then blend until smooth. Chill mixture and reserve. Can be made a day or two in advance.
For the ribs: Remove membrane from back of the spareribs with a sharp paring knife and a kitchen towel. Brush both sides lightly with oil and evenly sprinkle on dry rub (ingredients at left), starting with the back. Cover and cure for at least 3 hours and up to 12 hours in the refrigerator.
When ready to cook, allow to temper at room temperature for 1 hour. Season both sides with kosher salt. Set up grill for indirect cooking: Heat one side to medium-low, leaving the other half cool. Place ribs over cool side and cook 20 minutes, flipping occasionally to balance the time on each side. The goal is to add grill flavor and a little caramelization before cooking completely. Once grilled, transfer ribs to a sheet tray with a rack. Allow to rest 15–20 minutes.
Preheat oven to 275°F. Use convection mode if you have it. Liberally and evenly brush barbecue sauce on both sides of the ribs, starting with the back. Bake with a high fan for 1 1⁄2 hours or until pork reaches an internal temperature of 180-190°F. Remove from oven, brush with more sauce, cover with a foil tent, and allow to rest 30–40 minutes before serving.