Melt the butter in a medium saucepan over medium-low heat. Add the garlic and saute until it is fragrant and light golden brown, about 1 minutes.
Stir in flour and whisk continuously for 1 minute. Slowly whisk in the chicken broth. Increase the heat to a simmer and whisk continuously for several minutes, until the sauce is smooth and thickened. Remove saucepan from heat, and stir in chili powder, cumin, garlic powder and salt. Add black pepper to taste and set sauce aside.
Place the chicken breasts in a large slow cooker. Top them with the diced tomatoes with green chiles and jalapeno. Pour the sauce over the top. Cover slow cooker and cook on low for 4- hours, until chicken is tender and cooked through, but not overcooked.
Transfer chicken to cutting board, Stir cheese into the slow cooker and cover the slow cooker while you prepare chicken. Pull chicken into large chunks and stir in back into the slow cooker.
Cover the slow cooker and cook for 15 minutes, until the cheese is melted and the ingredients heated through. Stir well.
To serve, crush tortilla chis into a bowl and ladle the soup over the top. Garnish with extra cheese and cilantro, if desired.