Place the chicken breasts in the bottom of a 6-quart slow cooker. Add the chicken broth and ½ cup buffalo sauce. Sprinkle the garlic powder, onion powder, kosher salt, and black pepper over the chicken.
Cover and cook on low for four to five hours, or until the chicken is tender and reaches an internal temperature of 165°F.
Transfer the chicken to a plate and shred it with two forks. Leave the cooking liquid in the slow cooker, then return the shredded chicken to the crockpot.
Cook the elbow macaroni in a large pot of salted boiling water until just al dente according to the package directions. Drain well and set aside.
Add the butter, softened cream cheese, evaporated milk, and remaining ½ cup buffalo sauce to the shredded chicken. Stir, cover, and cook on high for 10 to 15 minutes, or until the cream cheese has softened and blends easily into the sauce.
Add the shredded cheddar and Monterey Jack cheese a handful at a time, stirring after each addition until melted.
Add the drained macaroni and gently stir until the pasta and chicken are completely coated in cheese sauce.
Cover and cook on high for another 10 to 15 minutes, just until everything is hot and creamy. Switch the crockpot to warm.
If the sauce becomes too thick, stir in two to four tablespoons of warm evaporated milk. Taste and add additional buffalo sauce if desired.
Serve hot with sliced green onions, crumbled blue cheese, or another drizzle of buffalo sauce.