In a large glass measuring cup, whisk together milk, pepperoncini brine, yogurt, mayonnaise, and dry ranch seasoning. Stir in pepperoncinis. Cover and chill at least 20 minutes.
Preheat grill to medium-high heat.
Combine Challenge Spreadable Butter with Canola Oil, dry ranch seasoning and chives in a small bowl. Mix until smooth. Spread each chicken filet with a spoonful of ranch butter; reserve the rest for after cooking.
Place chicken on grill and cook until dark grill marks develop, about 5 minutes.
Flip and cook another 3-5 minutes, until chicken registers 165° F. Transfer to a cutting board, tent loosely with foil, and let rest 5 minutes. Slice the chicken, spread with another spoonful of ranch butter, and set aside.
Combine romaine, corn, cucumber, and tomatoes in a large bowl. Toss with half of the zesty ranch dressing. Top with sliced chicken and avocado. Serve with more dressing on the side.
