How to Make It
In a medium cast-iron casserole, heat the olive oil. Add the chicken and cook over high heat until nicely browned, about 2 minutes per side. Add the cumin and 1 cup of water; season with salt and bring to a simmer. Cover and cook over low heat until the chicken pulls apart easily with a fork, about 1 hour. Transfer the chicken to a cutting board or bowl and shred, then return to the cooking liquid.
Meanwhile, in a small saucepan, combine the black beans with the barbecue sauce, the chile powder, cayenne and 1/2 cup of water; season with salt. Bring to a simmer, then cover and cook over low heat until the beans are very soft and the sauce is thick, about 30 minutes.
On a microwave-safe plate, microwave the tortillas at high power for 30 seconds, until soft and pliable. Spoon the beans on the tortillas, top with the chicken, avocado, and cilantro leaves and serve.