Preheat oven to 400 degrees F (200 degrees C). Lightly grease a 9x13 inch baking dish.
Place the potatoes in the baking dish. Season with chili powder, salt, and pepper. Evenly distribute the butter over the potatoes.
Cover dish with aluminum foil, and bake 1 hour in the preheated oven, until potatoes are tender. Remove from oven, and mix in the cheese and Ranch dressing. Continue cooking 10 minutes, or until cheese is melted and bubbly.