Heat oven to 300°F (150°C). Remove the membrane from the bone side of each rack. Mix together the 2 tbsp brown sugar, 1 tbsp smoked paprika, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp kosher salt, 1/2 tsp black pepper, and 1/2 tsp cayenne. Pat ribs dry and press the rub firmly onto both sides.

Wrap each rack tightly in heavy-duty foil, place on a rimmed baking sheet, and bake at 300°F for 2 hours and 30 minutes.

While ribs bake, combine the 1 cup ketchup, 1/4 cup apple cider vinegar, 3 tbsp brown sugar, 2 tbsp Worcestershire sauce, 1 tbsp yellow mustard, 1 tsp smoked paprika, 1/2 tsp garlic powder, and 1/2 tsp liquid smoke in a saucepan over medium heat. Stir until sugar dissolves, then simmer on low for 15 minutes.

Set oven to broil on high. Unwrap ribs, place meat-side up on the baking sheet, brush with half the BBQ sauce, and broil 6 inches from the element for 4 to 5 minutes until the glaze bubbles and edges begin to char.

Brush racks with another coat of BBQ sauce, slice between the bones, and arrange on a platter with the remaining sauce on the side and a scatter of sliced scallions on top.