Preheat oven to 400°F (200°C). Toss halved potatoes with 1 tbsp olive oil, garlic powder, salt, and pepper; spread in a baking dish.
Pat chicken dry, rub with remaining oil, then coat evenly with ranch seasoning; place on top of potatoes.
Mix sour cream and melted butter; dollop over potatoes and around chicken. Cover with foil.
Bake covered 25 minutes, remove foil, sprinkle cheddar over chicken and potatoes, bake uncovered 10–12 minutes until chicken reaches 165°F and cheese is bubbly.
Let rest 5 minutes, garnish with parsley, slice and serve family-style.