Cut the chicken into strips or bite-size chunks. Place in the bottom of the slow cooker; sprinkle with salt and pepper.
Whisk together chicken broth, milk, corn starch, sugar and garlic powder. Pour over the chicken. Top with the butter.
Cover and cook on high heat for 3-4 hours or low heat for 7-8 hours.
In a separate bowl, combine sour cream, Greek yogurt and flour until smooth. Stir into the the cooked chicken mixture and continue cooking for an additional 15 minutes until thickened.
Taste for seasoning, adding more salt as necessary. Serve hot over egg noodles.
Notes
You may use all sour cream or all Greek yogurt instead of half of each if preferred. Although my family serves this alongside vegetables, you could easily add vegetables into it. To add mushrooms, add sliced or diced mushrooms with the chicken in step 1. For other vegetables such as peas, add thawed frozen peas with the flour and sour cream mixture in step 4. This will allow them to heat through without getting mushy.