Preheat oven to 325 degrees F. Place ribs in two 15x10x1-inch baking pans or roasting pans.
In a small bowl, combine paprika, garlic powder, 2 tablespoons brown sugar, cumin, onion powder, chili powder, 2 teaspoons salt, and cayenne pepper. Sprinkle ribs evenly on both sides with the spice mixture, rubbing in with your fingers. Pour 1 cup of water into the bottom of each baking pan and cover the pans tightly with foil.
Place the covered baking pans in the oven and bake 2 hours or until a knife can pierce meat with no resistance.
Meanwhile, make the Plum BBQ Sauce. In a medium saucepan, heat the olive oil over medium-high heat. Add the onion and garlic and cook and stir 2 to 3 minutes or until softened and fragrant. Add the plums, tomato sauce, 2 tablespoons brown sugar, Worcestershire sauce, vinegar, chipotle pepper and adobo sauce. Bring to boiling. Reduce heat and simmer, uncovered, 20 minutes or until thickened. Remove from heat and let cool slightly. Transfer to a blender or food processor. Cover and blend or process until smooth. Season to taste with salt.
Remove the ribs from oven. Discard liquid from the baking pans. Brush the ribs with half of the Plum BBQ Sauce.
For a gas or charcoal grill, place the ribs on the rack of a covered grill directly over medium heat. Grill 2 minutes per side or until sauce is bubbling and ribs are lightly charred. Cut into 2-rib portions. Serve immediately with the remaining Plum BBQ Sauce. Garnish with additional plums, if desired.