Preheat the oven to 350 degrees F (175 degrees C).
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 fork-tender, 5 to 8 minutes.
Meanwhile, beat cream cheese and milk together in a bowl. Stir in Parmesan and Cheddar cheeses.
Mix bread crumbs and melted butter together in a small bowl.
Transfer steamed cauliflower to a 2 1/2-quart casserole dish. Add cheese-milk mixture and stir. Sprinkle with bread crumb mixture.
Bake in the preheated oven until bubbly and browned, about 20 minutes.