In a small bowl, combine the brown sugar, salt, mustard, garlic powder, black pepper, cayenne and paprika. On 2 baking sheets, sprinkle the spice mix all over the ribs, pressing and patting it into the ribs. Cover with foil and refrigerate for at least 4 hours, overnight if possible.
Preheat the oven to 250 degrees F. Keep the ribs covered in foil and roast for 3 hours, until the meat is super tender but not falling off the bone. Remove from the oven and pour off any liquid on the baking sheets.
While the ribs are cooking, make the BBQ sauce. in a saucepan, melt the butter over medium high heat. Add the onion, garlic and thyme and cook over medium heat until the onion is translucent, about 5 minutes. Add the ketchup, vinegar, beef broth, Ccholula Hot Sauce, Worcestershire sauce and molasses and bring to a boil. Reduce to a simmer, and stir occasionally, until thick, about 30 minutes. Remove from the heat and set aside.
Preheat the broiler or BBQ. Brush the ribs liberally with the barbecue sauce and broil or grill for about 10 minutes, turning and brushing occasionally with the sauce, until well-browned and crispy in spots. Transfer the ribs to a clean cutting board and let rest for 5 minutes. Cut in between the bones and mound the ribs on a platter. Pass any extra barbecue sauce on the side and garnish with green onions