1. Preheat the oven to 350°. In a medium heatproof bowl, cover the ancho chiles with the boiling water and let soak until softened, about 20 minutes. Drain the anchos and reserve the soaking liquid. Stem, seed and coarsely chop the anchos.
2. In a food processor, pulse the anchos with the garlic, vinegar, cumin, cinnamon, allspice, cayenne and 1 teaspoon of salt until a coarse paste forms. Add the reserved ancho soaking liquid and process until smooth.
3. Arrange the chicken legs in a shallow roasting pan, skin side up. Pour the ancho sauce on the chicken and toss to coat. Roast the chicken legs in the upper third of the oven for 1 1/2 hours, or until the meat is cooked through and the skin is crisp. Using tongs, lift the crisp skin off each chicken leg in 1 piece and transfer it to a baking sheet. Flatten each piece of skin with your hands. Season the chicken skin with salt and pepper, then bake for 2 to 3 minutes, or until very crisp.
4. Transfer the chicken legs to a large plate. Remove the meat from the bones. Shred the chicken and spread it in a medium roasting pan. Pour the roasting juices into a glass measuring cup and skim the fat from the surface. Pour the juices over the chicken and season with salt and pepper.
5. Preheat the broiler. Broil the shredded chicken for about 4 minutes, stirring a few times, until the chicken begins to crisp at the edges. Transfer the chicken and sauce to a warmed serving bowl. Arrange the crisp chicken skin in another bowl and serve with the warmed tortillas.
Make Ahead: The recipe can be prepared through Step 4 and refrigerated for 1 day. Crisp the skin in a 400° oven for 5 minutes. Bring the chicken to room temperature before broiling.
Serve With: Diced avocados, shredded iceberg and romaine lettuces, lime wedges, shredded Jack cheese, finely shredded Asiago cheese and crumbled soft goat cheese.
Wine Recommendation: The sweet and hot spices in this dish call for a simple, direct, fruity wine. Look for a fresh, inexpensive rosé with good acidity, such as Château Routas Rouvière Rosé from Provence or Preston of Dry Creek Le Petit Faux Rosé Dry Creek Valley.