In the slow cooker, whisk together the cream of chicken soup, dry chicken gravy mix, water or chicken broth, garlic powder, and ground black pepper. Add the chicken breasts and stir to coat in the sauce. Place the butter slices on top of the chicken. Cover and cook on low for 4-6 hours, until the chicken reaches an internal temperature of 165°F (74°C). Stir in the sour cream 30 minutes before serving. Let the chicken rest for 5 minutes, then shred it into bite-sized pieces. Stir the chicken back into the sauce. Serve over mashed potatoes, and garnish with parsley.