Ribs
2 tablespoons paprika
1 tablespoon packed brown sugar
1 tablespoon kosher salt
2 teaspoons pepper
2 teaspoons onion powder
2 teaspoons granulated garlic
5 - 6 pounds pork spareribs (2 racks, 2 1/2- to 3-pounds each), preferably St. Louis-style, trimmed and each rack cut in half
Barbecue Sauce
¾ cup ketchup
6 tablespoons apple juice
2 tablespoons molasses
2 tablespoons cider vinegar
2 tablespoons Worcestershire sauce
1 tablespoon yellow mustard
1 teaspoon pepper
¼ teaspoon liquid smoke
FOR THE RIBS: Combine paprika, sugar, salt, pepper, onion powder, and granulated garlic in bowl. Reserve 1 tablespoon spice rub for sauce. Pat ribs dry with paper towels and coat all over with remaining 5 tablespoons rub.
Arrange ribs vertically with thick ends pointing down and meaty side against interior wall of slow cooker (ribs will overlap). Cover and cook until ribs are just tender, 5 to 6 hours on high or 6 to 7 hours on low.
FOR THE BARBECUE SAUCE: Meanwhile, whisk ketchup, apple juice, molasses, vinegar, Worcestershire, mustard, pepper, liquid smoke, and reserved 1 tablespoon spice rub together in medium saucepan. Bring to boil over medium heat, then reduce heat to medium-low and simmer, stirring occasionally, until thickened and reduced to 1 cup, about 10 minutes. (Sauce can be refrigerated for up to 3 days.)
Line rimmed baking sheet with aluminum foil and set wire rack in sheet. Using tongs, transfer ribs, meaty side up, to prepared rack. Let ribs sit for 10 minutes to allow surface to dry out.
Adjust oven rack 3 inches from broiler element and heat broiler. Liberally brush ribs with 1/2 cup sauce and broil until sauce is bubbling and beginning to char, about 4 minutes. Remove ribs from oven, brush with remaining ½ cup sauce, tent with foil, and let rest for 20 minutes. Cut ribs in between bones to separate. Serve.