1. Line 2 rimmed baking sheets with foil, and place a wire rack on top of each. Transfer 2 racks of ribs to each wire rack, meat side up.

2. Using a sieve, sift sugar, paprika, salt, cocoa, cayenne, and cumin together into a small bowl. (Use the back of a spoon to break up any lumps and to press ingredients through a sieve.) Stir to combine.

3. Sprinkle spice mixture evenly over both sides of each rack of ribs. Lightly rub mixture into the ribs to coat completely. Cover with plastic wrap. Refrigerate on racks on sheets at least 20 minutes (or up to 2 hours for maximum flavor).

4. Preheat oven to 350 degrees, with racks in upper and lower thirds. Cook ribs on racks on sheets, switching positions of baking sheets about halfway through, until meat pulls away easily from the bones, 1 1/2 to 2 hours. Let cool completely. Refrigerate, covered, until ready to finish cooking, up to 2 days. (If finishing cooking immediately, raise oven temperature to 425 degrees and continue with step 6.)

5. Preheat oven to 425 degrees. Let ribs stand at room temperature 20 minutes.

6. Using 2 cups reserved barbecue sauce, brush both sides of ribs. Cook ribs on wire racks set over rimmed baking sheets lined with foil until sauce is slightly caramelized and ribs are heated through, about 15 minutes. Let stand 5 minutes; serve with more barbecue sauce.