1. In a small bowl, mix the brown sugar, paprika, sugar, celery salt, Kosher salt, onion powder, garlic powder, chili powder, white pepper and black pepper.
2. Using a sharp knife, carefully remove the membrane from the back of the rack of ribs. Trim the ribs of excess fat. Rub each rack generously with the rub.
3. Place the ribs in a large container and refrigerate overnight.

To prepare the barbecue sauce:

1. In a medium saucepan over medium heat, melt the butter. Add the onion. Cook until softened, about 5 minutes. Add the garlic, and cook until fragrant, about 1 minute.
2. Add the ketchup, molasses, brown sugar, vinegar, mustard, chili powder, black pepper and cayenne pepper. Stir to combine. Bring the mixture to a boil, then reduce heat and simmer. Stir frequently until thickened, about 30 minutes.
3. Using a food processor, blend sauce until smooth. Allow to cool to room temperature.

To prepare the ribs:

1. Prepare an outdoor grill to a heat of 225 degrees F. Place the ribs on the grill, meat side up, until the ribs are cooked through, about 5 hours.
2. During the last 30 minutes of cooking, generously baste each rack with the barbecue sauce.