Step 1

Line a large heavy rimmed baking sheet with foil and put rib racks side by side in pan.

Step 2

In a bowl, whisk together 1/4 cup brown sugar, chili powder, paprika, salt, pepper, and cayenne and rub onto both sides of racks. Let ribs stand, meaty sides up, at room temperature, 1 hour.

Step 3

Preheat oven to 325°. Cover pan tightly with foil and bake ribs 1 1/2 hours.

Step 4

Make sauce while ribs bake: Cook onion in oil in a 3-4-quart heavy pot over medium-low heat, covered, stirring occasionally, until tender, 10-15 minutes. Add garlic and cook, stirring, 1 minute. Add remaining ingredients, including remaining 1/3 cup brown sugar, and simmer, uncovered, stirring occasionally, 15 minutes. Cool sauce slightly and purée in a food processor. Set aside 1 cup sauce for serving.

Step 5

Remove foil cover and turn racks over so bone ends curve up, then baste ribs with pan juices and spread generously with some sauce. Continue to bake, uncovered, 30 minutes. Turn racks over, then baste again with pan juices and top generously with more sauce. Continue to bake until tender when pierced with a small knife, 30-45 minutes.

Step 6

Broil ribs, meaty sides up, 4"-6" inches from heat, until browned, 2-3 minutes. Serve with reserved sauce.