Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower, cover, and steam until very tender, 15 to 20 minutes. Transfer cauliflower to a bowl, mash, and strain excess water.
Mix cauliflower, sour cream, Cheddar cheese, ranch dressing mix, onion powder, and garlic powder together in a 9-inch baking dish; top with butter.
Bake in the preheated oven until bubbling, 30 to 45 minutes.