1/2cplus 2 tablespoons packed dark brown sugar, divided
1cplus 2 tablespoons balsamic vinegar, divided
1tsp.freshly ground black pepper
8lbbaby back pork ribs
1cwater plus 2 cups hot water
Mince and mash garlic to a paste with 1 teaspoon of salt. Stir together with the rosemary, 2 tablespoons of the brown sugar, 2 tablespoons of the vinegar, cayenne, remaining 3 teaspoons of salt, and pepper. Rub evenly all over the ribs and transfer to 2 large roasting pans in a single layer, meaty side up. Marinate, chilled, for at least 8 hours and up to 24 hours. Alternatively marinate in a zippered bag or bowl covered with plastic wrap.
Preheat the over to 425 degrees Fahrenheit with racks in the upper and lower thirds of the oven.
Pour 1/2 cup water into each roasting pan and tightly cover the pans with foil. Roast the ribs, switching the position of the pans halfway through, until the meat is very tender, about 1 3/4 hours. Remove the pans from the oven and transfer the ribs to a platter.
Add 1 cup hot water to each roasting pan and scrape up the brown bits. Skim off and discard the fat, then transfer the liquid to a 10-inch skillet. Add the remaining 1 cup vinegar and 1/2 cup brown sugar and bring to a boil, stirring occasionally. Boil until thick and syrupy and reduced to about 1 cup, about 15 minutes.
In a charcoal grill, prepare a fire for direct-heat cooking over medium-hot charcoal. Alternatively, preheat a gas grill to medium heat. The ribs can also be broiled 3 to 4 inches from heat (instead of grilled) for about 8 minutes.
Brush some of the glaze onto both sides of the racks of ribs. Grill, turning occasionally, until the ribs are hot and grill marks appear, about 6 minutes. Brush the ribs with more glaze and serve with the remaining glaze on the side.