I copied this verbatim from "Zarela Martinez - Food from My Heart (Cuisines of Mexico Remembered and Reimagined)", a cookbook/memoirs that I picked up in a bargain bookstore. I am in the middle of preparing this very recipe and so far it smells heavenly. I hope that y'all will like it too.
I ate this with delicious pork chops at a hotel in Merida. I often substitute country-style ribs here. The adobo can also be used to coat a pork loin roast (baste frequently with pan juices). I have used it with great results on baked chicken wings.
If you have no ancho chiles, substitute any dried chile except chile arbol or chipotle. For a hotter flavor, leave in the seeds and veins.
Rinse the chiles under cold running water, removing the seeds and veins. Heat a griddle or cast-iron skillet over high heat until a drop of water sizzles on contact. Place the chiles on the griddle and heat until their aroma is released, 1 to 2 minutes on each side. Do not let them burn or they will make the dish horribly bitter. Place the toasted chiles in a small bowl or saucepan, cover with boiling water and let soak until somewhat softened, about 10 minutes.
Place the softened chiles, Achiote paste, orange juice, garlic and oregano in a blender or food processor fitted with the steel blade and process for 1 minute. Scrape down the sides with a rubber spatula, add the vinegar and process again until smooth. Taste for seasoning; add a little sugar, if desired, and the salt.
Rub the ribs or pork chops with the marinade and let rest in the refrigerator, covered, overnight (or for at least 4 hours).
Preheat the oven to 350F. Choose a roasting pan large enough to allow a little space between the pieces and bake the ribs or chops until no pink shows in the center, 20 to 30 minutes.