1. To make the Barbecue Sauce: Heat butter in small nonreactive saucepan over medium heat until foaming; add onion and cook, stirring occasionally, until softened, 2 to 3 minutes. Add garlic and chili powder; cook, stirring constantly, until fragrant, about 20 seconds. Add tomato juice, 1/2 cup vinegar, Worcestershire sauce, mustard, chipotle, molasses, and salt; increase heat to high and bring to simmer, then reduce heat to medium and continue to simmer, stirring occasionally, until sauce is slightly thickened and reduced to 1 1/2 cups, 30 to 40 minutes. Off heat, stir in pepper and remaining 1/4 cup vinegar. Cool to room temperature before serving. (Can be refrigerated in airtight container for up to 4 days; bring to room temperature before serving.)
2. For ribs:Mix chili powder, cayenne, salt, and pepper in small bowl; rub ribs evenly with spice mixture. Let ribs stand at room temperature for 1 hour.
3. Meanwhile, cover 2 large wood chunks (see note) with water and soak 1 hour. Drain wood chunks. Open bottom grill vents. Using chimney starter, ignite 30 briquettes (about one-third large chimney, or 2 quarts) and burn until covered with thin coating of light gray ash, about 10 minutes. Empty coals into grill, then bank coals against one side of grill, stacking them 2 to 3 coals high; place 1 soaked wood chunk on top of coals. Position grill grate over coals, cover grill, and adjust lid vents two-thirds open. Heat grate until hot, about 5 minutes; scrape grill grate clean with grill brush. Position ribs, meat-side down, on cool side of grill (they may overlap slightly); cover, positioning lid so that vents are directly above ribs. (Temperature on thermometer inserted through vents should register about 300 degrees.) Cook until grill temperature drops to about 250 degrees, about 1 hour. (On cold, windy days, temperature may drop more quickly, so spot-check temperature. If necessary, add 5 additional briquettes to maintain temperature above 250 degrees during first hour of cooking.)
4. After 1 hour, add 20 more briquettes and remaining wood chunk to coals; using tongs, flip ribs meat-side up and rotate so that edges once closest to coals are now farthest away. Cover grill, positioning lid so that vents are opposite wood chunk; continue to cook until dinner fork can be inserted into and removed from meat with little resistance, meat pulls away from bones when rack is gently twisted, and meat shrinks 1/2 to 1 inch up rib bones, 1 1/4 to 1 3/4 hours longer. Transfer ribs to cutting board and let rest 5 minutes; using chef's knife, slice between bones to separate into individual ribs. Serve, passing sauce separately.
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