In a bowl, stir together the paprika, chili powder, cayenne, garlic powder, salt, black pepper, white pepper, brown sugar and mustard powder. Place the ribs on a baking sheet and rub them evenly on both sides with the spice mixture. Wrap with plastic wrap and refrigerate overnight.
To make the mop sauce, in a bowl, whisk together the vinegar, brown sugar, hot sauce, Worcestershire sauce, water and salt. Set aside until ready to use.
Preheat a wood pellet grill on the “smoke” setting to 290°F according to the manufacturer’s instructions.
Prick the potatoes all over with a fork and wrap them individually in aluminum foil. Place the potatoes along the back side of the grill. Place the ribs, meaty side up, in the center of the grill. Cover the grill and cook until the potatoes and ribs are tender, about 3 hours; turn the potatoes over after 1 1/2 hours, and baste the ribs with the mop sauce every 5 minutes during the last 20 minutes of cooking. Transfer the ribs to a carving board and cover loosely with foil.
Remove the foil from the potatoes and cut each potato in half lengthwise. Scoop the flesh into a bowl, leaving a shell 1/8 inch thick. Place the potato shells on a grill tray. Pass the potato flesh through a potato ricer into a large bowl. Add the butter, sour cream and milk and stir until the butter is melted. Add 8 oz. of the cheese and the 1/4 cup chives and stir until incorporated. Season with salt and black pepper. Divide the mixture evenly among the potato shells and sprinkle with the remaining 2 oz. cheese.
Place the grill tray on the grill, cover and cook until the cheese has melted, about 10 minutes. Garnish the potatoes with the 1 Tbs. chives. Cut the ribs between the bones and serve immediately with the twice-baked potatoes. Serves 6 to 8.