Transfer to slow cooker, ribs side down. Rub butter mixture on top and sides of roast. Cover and cook on Low heat setting 3 to 5 hours or until desired temperature in center. For medium-rare, cook 3 hours 30 minutes to 4 hours, or until meat thermometer inserted in center reads 135°F. For medium, cook 4 hours 30 minutes to 5 hours, or until meat thermometer inserted in center reads 145°F. Transfer roast to cutting board. Cover roast loosely with foil; let stand 30 minutes before carving. Cut into slices.