Carefully remove the thin membrane from the back of each rack of ribs. To do this, slide a small knife under the membrane at one end, then grip it with a paper towel and pull it off. This step is crucial for tender ribs.

Cut each rack of ribs into 2-3 sections so they fit comfortably into your slow cooker. You might need to arrange them upright or curved.

In a small bowl, combine all the dry rub ingredients: brown sugar, smoked paprika, garlic powder, onion powder, chili powder, salt, black pepper, and cayenne pepper (if using). Mix well.

Generously rub the mixture all over both sides of the rib sections, pressing it in gently to adhere.

Arrange the seasoned rib sections in your slow cooker. It's okay if they overlap slightly, but try not to pack them too tightly.

Pour the apple cider vinegar, water, and liquid smoke (if using) into the slow cooker around the ribs.

Cover and cook on HIGH for 4 hours or on LOW for 7-8 hours, or until the ribs are very tender and almost falling off the bone.

Once cooked, carefully remove the ribs from the slow cooker and transfer them to a foil-lined baking sheet. Discard the liquid from the slow cooker.

Preheat your broiler to high.

Brush a generous layer of your favorite BBQ sauce over both sides of the ribs.

Place the baking sheet under the broiler for 3-5 minutes, or until the sauce is bubbly and slightly caramelized. Keep a close eye on them to prevent burning. Flip and baste with more sauce, then broil for another 2-3 minutes if desired.

Remove from the oven, let rest for a few minutes, then slice between the bones and serve hot.