Rinse and drain beans and place in the bottom of a large (7- to 8-quart) slow cooker.
Lay chicken breasts on top, and add garlic, green chiles, and jalapeño (if using) to the slow cooker.
Carefully pour chicken broth over ingredients, and season with cumin, paprika, cayenne (if using), and black pepper.
Cook on low for 8 to 10 hours or until beans are tender.
An hour before serving, remove chicken to a plate or cutting board.
Mix masa into warm milk, stir mixture into white chili, and replace lid of slower cooker.
Cut chicken into large chunks and shred it with two forks.
Add chicken back to the white chili and taste for seasoning, adding salt and adjusting spices as necessary.
Cover and continue to cook for one more hour.
Before serving, garnish with a generous amount of cilantro, crushed tortilla chips, and shredded cheese, plus whatever other toppings appeal to you.
