Melt the butter in a medium saucepan over medium-low heat. Add the garlic and saute until it's fragrant and light golden brown, about 1 minute. Stir in the 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 the saucepan from the heat, and stir in the chili powder, cumin, garlic powder and salt. Add black pepper to taste and set the sauce aside.
Place the chicken breasts in a large slow cooker. Top them with the diced tomatoes with green chiles and jalapeño and pour the sauce over the top. Cover the slow cooker and cook on low for 4 to 6 hours, until the chicken is tender and cooked through but not overcooked.
Transfer the chicken to a cutting board, stir the cheese into the slow cooker and core the slow cooker for the time being.
Pull the chicken into large chunks and stir it back into the slow cooker. Cover the slow cooker and cook for 15 minutes, until the cheese is melted and all of the ingredients are heated through. Stir well.
To serve, crush the tortilla chips into a bowl and ladle the soup over the top, garnishing with extra cheese and the cilantro, if desired.