In a mixing bowl, whisk together cream of chicken soup, chicken broth, onion soup mix, garlic powder, and black pepper until smooth.
Place the chicken breasts in the bottom of the crockpot. Pour the gravy mixture over the top, ensuring the chicken is fully coated. Add butter on top if desired.
Cover and cook on LOW for 4 to 5 hours or on HIGH for 2½ to 3 hours, until the chicken is tender and easily shreds with a fork.
Remove the chicken and shred with two forks. If a thicker gravy is desired, whisk the cornstarch and water together, stir into the sauce, and cook uncovered for another 10 to 15 minutes until thickened.
Return the shredded chicken to the crockpot, stir gently to coat with gravy, and serve warm over mashed potatoes, rice, or egg noodles. Garnish with fresh parsley.