Spicy and Sticky Baby Back Ribs

Name: Spicy and Sticky Baby Back Ribs
Ingredients: 1 cup dark brown sugar; 3 tablespoons kosher salt; 1 tablespoon dry mustard; 1 tablespoon ground fennel; 1 tablespoon freshly ground black pepper; 1 tablespoon cayenne pepper; 1 tablespoon sweet smoked paprika; 4 racks baby back ribs (about 2 1/2 pounds each), membrane removed from the underside of each rack; 1 tablespoon unsalted butter; 1 small onion, minced; 3 garlic cloves, minced; 1 1/2 teaspoons dried thyme; 1 cup ketchup; 1 cup cider vinegar; 1 cup beef broth; 1/4 cup hot sauce; 1/4 cup Worcestershire sauce; 2 tablespoons unsulfured molasses