Place the boneless, skinless chicken breasts at the bottom of the slow cooker.
In a mixing bowl, combine the black beans, corn, diced tomatoes (with their juice), tomato sauce, chopped onion, minced garlic, chili powder, cumin, salt, black pepper, and cayenne pepper (if using). Stir to combine thoroughly.
Pour the bean and tomato mixture over the chicken in the slow cooker, ensuring the chicken is well covered with the mixture.
Cover the slow cooker and cook on low for 6 hours, or until the chicken is fully cooked and tender.
Once cooked, carefully remove the chicken from the slow cooker and shred it using two forks.
Return the shredded chicken to the slow cooker and stir to combine with the remaining mixture.
To serve, spoon the chicken mixture onto a large flour tortilla, sprinkle with shredded cheese, and fold it up like a burrito.
Optionally, serve with your favorite toppings like sour cream, avocado, salsa, cilantro, and lime wedges.