Whisk broth, pepperoncini brine, buttermilk powder, parsley, dill, garlic powder, onion powder, salt and pepper in a 5-quart slow cooker.
Nestle whole chicken breasts into the liquid and add butter pieces and pepperoncini around them. Cover and cook on low for 3 to 4 hours, checking from 3 hours and removing breasts as they reach 165°F.
Cover cooked chicken on a clean plate. Pour the cooking liquid into a saucepan, leaving the pepperoncini aside with the chicken.
Mix cornstarch with cold water and whisk into the simmering liquid. Cook for 2 to 3 minutes until lightly thickened.
Serve one whole breast per person with 2 pepperoncini and the tangy herb sauce spooned over the top.