Sweet-and-Spicy Ribs

Preheat oven to 325 °F. Combine brown sugar, paprika, cayenne, garlic powder, and 3 tablespoons salt in a small bowl; set aside 1/4 cup spice mixture. Pat ribs dry with paper towels and rub with remaining spice mixture, massaging into crevices. Wrap each rack in a double layer of foil, crimping edges to seal tightly. Place each foil packet on a rimmed baking sheet and bake until meat is fork-tender at the thickest part but not falling off the bone, 2 –3 hours. Let cool. Tear open a corner of each foil packet and carefully pour juices from ribs into a large heatproof measuring cup. Let juices settle, then spoon fat from surface and discard. Heat butter in a medium saucepan over medium, stirring often, until it foams, then browns (do not burn), 5 –8 minutes. Add juices from ribs and 1/4 cup reserved spice mixture and bring to a simmer. Cook until reduced by one-third, 15 –20 minutes. Remove sauce from heat, add vinegar, and season with salt. Prepare grill for medium-high heat; oil grate. Remove ribs from foil and grill, basting with sauce and turning occasionally, until charred in spots, 5 –7 minutes. Transfer to a cutting board and let rest 10 minutes. Slice between ribs and serve with remaining sauce.