3/4 cup (packed) light brown sugar
1/4 cup smoked paprika
2 tablespoons cayenne pepper
2 tablespoons garlic powder
3 tablespoons kosher salt, plus more
2 St. Louis-style pork ribs (3–4 pounds each)
1/2 cup (1 stick) unsalted butter
1/2 cup apple cider vinegar
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.
Ribs can be baked 1 day ahead; let cool in foil, then chill. Bring to room temperature before grilling. Sauce can be made 1 day ahead; cover and chill. Reheat over low.