Begin by trimming the ribs of any excess fat or sinew. Remove the thin membrane from the back side of the ribs and discard. Make the rub by combining the cumin, garlic, onion powder, paprika, salt, pepper and ancho chile powder together in a mixing bowl.
Set a few large sheets of foil out (one set for each slab). Place 1 rack on each sheet and divide the dry rub between each slab. Rub the mixture into the ribs on both sides, and then fold the foil over to form a sealed pouch around each rack. At this stage make sure the racks are meat-side down. Place each pouch on a roasting tray and bake in the oven for 2 hours. After 2 hours, open up the foil and peel back so the ribs are exposed. Carefully turn the meat over so the meat side is on top, and then cook for 1 1/2 more hours with the open foil. When done, the ribs will be tender and the meat will have shrunk back from the bones. Finish under the broiler to add more color, 1 to 2 minutes.
Serve the ribs with Tequila BBQ Sauce.
Tequila BBQ Sauce:
Begin by placing the dried chile in a bowl with 1/4 cup hot water to rehydrate. In a medium saucepan, add the tomato paste, light brown sugar, tequila, lime juice, garlic and molasses. Stir well and bring to a gentle simmer. Add in the chile and soaking water, season with salt and pepper and simmer until the flavors meld together, 2 to 3 minutes. Process with a blender until smooth and serve with the ribs as a dipping sauce.