Add chicken to a 6-quart slow cooker. Season with poultry seasoning, salt, and pepper.
Add onion, celery, carrots, garlic, and bay leaves. Pour broth over the top.
Cook on LOW for 6–7 hours (or HIGH for 3–4 hours) until chicken is very tender.
Remove chicken, shred it, discard bones/skin and bay leaves, then return shredded chicken to the slow cooker.
Turn slow cooker to HIGH and let broth heat up for 10–15 minutes.
Stir in noodles. Cover and cook on HIGH until noodles are tender (frozen noodles 30–45 minutes; dry noodles 15–25 minutes), stirring once or twice to prevent clumping.
Stir in butter until melted. Add cream if using. Taste and adjust salt and pepper.
If you want it thicker, stir in the cornstarch slurry during the last 10 minutes of noodle cooking. Serve hot.