Marinade
3 ancho chiles, stemmed
3 chiles de árbol, stemmed
3 garlic cloves, lightly crushed
1 cinnamon stick, broken into pieces
1/2 teaspoon black peppercorns
1/2 teaspoon cumin seeds
1 1/2 cups water
1/2 cup cider vinegar
4 teaspoons light brown sugar
1 1/2 teaspoons achiote paste
1 teaspoon kosher salt
3/4 teaspoon dried Mexican oregano
Chicken Skewers
2 pounds boneless, skinless chicken thighs, cut into 1-inch pieces
8 (10-inch) skewers (soaked in water if wooden)
1 1/2 teaspoons kosher salt
8 fresh pineapple wedges (about 1 inch each)
Canola oil, for grilling
Warm corn tortillas, sliced avocado, fresh cilantro leaves, and lime wedges, for serving
Directions
Make the al pastor marinade
Toast ancho chiles in a medium saucepan over medium-low, turning occasionally, until fragrant and pliable, about 5 minutes. Add chiles de árbol, garlic, cinnamon, peppercorns, and cumin seeds; cook, stirring constantly, until chiles de árbol just start to brown and spices are fragrant, about 1 minute. Add 1 1/2 cups water; bring to a boil over high. Boil 5 minutes, stirring and turning chiles occasionally. Remove from heat, and let chile mixture stand 20 minutes.
Transfer chile mixture and cooking liquid to a blender; process until smooth, about 30 seconds. Add vinegar, brown sugar, achiote paste, salt, and oregano; process until smooth, about 30 seconds.
Reserve 1/4 cup al pastor marinade in a small bowl, and set aside. Place chicken and remaining marinade (about 1 1/2 cups) in a large ziplock plastic bag. Press out as much air as possible, and seal bag; shake bag to coat chicken evenly in marinade. Place bag on a large plate or rimmed baking sheet, and refrigerate at least 30 minutes or up to 1 hour.
Preheat a grill to medium-high (400°F to 450°F). Remove chicken from marinade, and place on a large plate or rimmed baking sheet; discard bag with marinade. Thread evenly divided chicken pieces onto skewers, pressing pieces together so that they are just touching; sprinkle with salt. Thread 1 pineapple wedge onto the end of each skewer.
Working in batches if necessary, place skewers on oiled grates, and grill, turning occasionally, until chicken is charred on all sides and cooked through, 12 to 15 minutes, brushing with reserved 1/4 cup marinade during final 3 minutes of grilling. Transfer skewers to a large platter. Serve hot with corn tortillas, sliced avocado, cilantro, and lime wedges.