Combine the chicken breasts, diced tomatoes, 1/2 cup of chicken stock, chili powder, cayenne pepper, salt, and cumin in the bowl of a 2 1/2- to 3 1/2-quart slow cooker. Make sure the chicken is covered. Cover with the lid and cook on low for 1-1/2 hours.
Remove the lid and add the black beans, corn, and another 1/2 cup of chicken broth. Replace the lid and continue cooking on high for another 1-1/2 hours. Turn slow cooker down to Low while making the rice. Total cooking time is 3 - 3-1/2 hours.
Remove chicken from slow cooker with tongs and place on a baking sheet. Use two forks to shred the chicken into bite-sized pieces. You can either return the chicken to the slow cooker or place in a bowl to serve. Taste the burrito mix and stir in more salt or other seasonings to taste.
Note: When slower cooker is finished cooking, there will be a fair amount of liquid in the slow cooker. I use a slotted spoon to spoon the veggies into the the individual bowls. You can also cook the rice directly in the slow cooker, but given how temperamental rice can be, its best to cook it on the stove top or microwave.
If you would like to cook the rice in the slow cooker make the following adjustments:
Drain tomatoes before adding to slow cooker
Increase amount of chicken stock to 1-1/4 Cup. Only add 1/2 C of chicken stock in the begging with chicken, tomatoes and seasonings
1/2 way through the cooking time, add in rice with the black beans and corn. Also add the other 3/4 Cup of chicken stock. Stir to cover rice.
After 3 hours of cooking, check rice for doneness. If it is too dry, add a little bit more chicken stock. If it is too wet, remove the lid from the slow cooker and continue cooking for another 30 mins or until liquid evaporates.