Preheat oven to 300 degrees. Trim ribs of excess fat and silver skin then season lightly with salt, pepper and garlic powder. Wrap each rack separately in a large piece of heavy-duty foil and seal edges. Bake 2½–3 hours or until very tender.

Meanwhile, mix all ingredients for barbecue sauce in a heavy-bottomed saucepan. Bring to a boil over medium heat then reduce heat to low. Simmer uncovered for 35–40 minutes, stirring occasionally until thickened. Remove sauce from heat and set aside to cool. Puree in blender or food processor until smooth.

When ribs are tender, remove from oven and increase oven temperature to 375 degrees. Remove ribs from foil and place them directly on baking sheet. Brush ribs generously on both sides with sauce then return to oven for 10–15 minutes or until sauce is set. Cut ribs apart and serve with remaining sauce on the side.