Combine the chicken breasts, diced tomatoes, 1/2 cup of chicken stock, chili powder, salt, and cumin in the bowl of a 2 1/2- to 3 1/2-quart slow cooker. Make sure the chicken is covered, and add additional stock if needed. Cover with the lid and cook on low for 3 to 4 hours.
Remove the lid and add the rice, black beans, frozen corn, and the remaining chicken broth. Replace the lid and continue cooking on low for another 3 to 4 hours. Check the rice periodically in the last hour of cooking, stirring once or twice to make sure the rice cooks evenly and adding more chicken broth if the mixture seems dry. Cooking is done when the rice is tender — if the rice is done while there is still liquid left in the slow cooker, remove the lid and cook on high to let the liquid evaporate.
→ Total cooking time from start to finish is 6 to 8 hours.
Use two forks to shred the chicken into bite-sized pieces. You can do this either in the slow cooker itself and then mix it into the rice, or you can transfer the chicken to a cutting board if you prefer to keep it separate. Taste the burrito mix and stir in more salt or other seasonings to taste.
Serve burrito bowls with a selection of toppings. The burrito mixture will keep for 1 week refrigerated or for 3 months in the freezer. This recipe makes about 7 total cups of burrito mix.