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 tender, 5 to 10 minutes. Transfer cauliflower to a bowl.
Mash cauliflower with cream cheese until smooth; stir in Parmesan cheese, garlic powder, salt, and black pepper. Spoon mixture into a 1-quart casserole dish; top with Cheddar cheese.
Bake in the preheated oven until bubbling and Cheddar cheese is melted, 10 to 15 minutes.