Combine water and onion in a pot and bring to a boil. Add chicken, garlic powder, and onion powder. Bring back to a boil and reduce heat to medium-low. Simmer broth until chicken thighs are tender, about 30 minutes.
Remove chicken and add carrots to the broth. Stir in 2 tablespoons fish sauce, chicken bouillon, and black pepper.
Shred chicken meat and discard bones; return meat to the broth and simmer until carrots are tender, about 10 minutes. Add macaroni; cook until tender, about 20 minutes, adding more water if needed. Stir in evaporated milk and butter; simmer until butter is melted. Add more fish sauce and season with salt and pepper to taste. Pour soup into serving bowls.