In a small bowl, stir together brown sugar, sweet paprika, smoked paprika, garlic powder, salt, pepper, dry mustard, and cayenne. Rub the mixture all over the ribs on both sides, coating evenly.
Arrange ribs (standing up) along the walls of a large (6 ½- to 7-quart) slow cooker, with the wide ends down and the meaty sides of the ribs facing outward. Evenly pour BBQ sauce over the tops of the racks. Cover slow cooker and cook on LOW for 6 to 8 hours, until ribs are tender.
Position oven rack about 10 inches from the broiler element and preheat broiler. Line a large baking sheet with heavy-duty aluminum foil and place a wire rack on top. Spray with non-stick cooking spray.
Use tongs and a metal spatula to transfer ribs to the wire rack-topped baking sheet. Lay them meaty side-down and tent with foil; set aside.
Remove fat from the surface of the braising liquid with a large spoon. Strain liquid through a mesh strainer into a medium saucepan. Set over high heat, bring to a boil, and reduce to a simmer. Simmer until liquid is thickened and reduced to 2 cups, anywhere from 10 to 20 minutes. Season with salt and pepper, to taste (only if necessary).
Remove foil tent from ribs, brush with sauce, and broil for 4 minutes. Carefully flip ribs over, brush with additional sauce, and continue to broil for 9 to 12 minutes or until well-browned and caramelized, brushing with more sauce every few minutes.
Tent ribs with foil once more and allow to rest for 10 minutes before transferring to a cutting board and slicing. Serve with remaining sauce.
Notes
I cooked two 2 ½-pound racks of baby back ribs for this recipe. They were perfectly cooked after 6 ½ hours in the slow cooker on LOW.
My favorite method for removing fat from the surface of the braising liquid is by laying a paper towel on top. After it soaks up the fat, I quickly lift it out and repeat with another paper towel until most of the fat is gone. I've read that you can achieve the same results by laying a slice of bread on the surface, but I haven't actually tried that method yet. ;)