Spray a slow cooker with cooking spray.
In a small bowl, stir together brown sugar, chili powder, salt, cumin, paprika, garlic powder, and onion powder. Set aside.
1 tablespoon packed brown sugar, 1 tablespoon chili powder, 1 tablespoon kosher salt, plus more as needed, ½ teaspoon ground cumin, ½ teaspoon smoked paprika, ½ teaspoon garlic powder, ½ teaspoon onion powder
Prepare the ribs by removing the shiny, thin membrane that covers most of the backside of the ribs. To do so, use a knife or fingernail to pry up a corner of the thin membrane from one edge of the ribs. Grab that with a paper towel (for grip) and slowly pull it up. Once a good portion of the membrane is free, you should be able to pull the rest off in one quick motion.
3 lbs. St. Louis-style pork ribs
Pat the ribs dry and rub the seasoning mixture onto both sides of the ribs. Place ribs in slow cooker.
Pour barbecue sauce and liquid smoke (if using) over the ribs. Cover and cook on LOW for 9-10 hours.
1 ½ cups barbecue sauce, plus extra for serving, 2 teaspoons liquid smoke, optional