Spray the insert of a large slow cooker with non-stick cooking spray. Remove ribs from packaging and pat dry with clean paper towel. If the membrane is still attached on the back of the racks, loosen it on one end by sliding a butter knife underneath, then grasp with a dry paper towel and pull the membrane off.
Mix all rub ingredients together in a small bowl. Rub generously into all sides of the rib racks. Place ribs in the slow cooker, cutting the racks into halves or thirds if needed.
Make the sauce: In a medium saucepan, whisk all sauce ingredients together. Bring to a simmer over medium heat, reduce heat slightly and allow to simmer until thickened, about 10-15 minutes, stirring frequently. Taste after about 10 minutes, add another tablespoon of brown sugar if you'd like it sweeter or add a teaspoon of apple cider vinegar if you'd like it more tangy.
Pour 1 cup of the sauce over the ribs. Cover and cook on low for 8 hours or until fork tender.
Preheat oven to 425℉. Place ribs on a foil lined sheet pan and brush with reserved sauce, bake for about 10 minutes, or until sauce is caramelized. You can also broil or grill the ribs. Be careful to not overcook the ribs. Cut into portions and serve immediately with remaining sauce.
Notes
Leftovers can be stored covered in the fridge for up to 5 days. These are best reheated in a 350℉ oven in a baking dish covered in foil until they reach 165℉.
For extra flavorful ribs, and to save time, you can use the dry rub a day ahead and let them marinate in the spice mix overnight. Then in the morning, continue with the recipe.
Don't skip the extra step of baking or broiling them after they come out of the slow cooker. This helps to caramelize the sauce and make them sticky and glazed.