Preheat the oven to 375°F (190°C). Grease a baking dish with butter or cooking spray.
Steam the cauliflower florets for about 8-10 minutes, or until just tender. Drain and set aside.
In a saucepan, melt butter over medium heat. Stir in the flour and cook for 1-2 minutes, until the mixture turns a light golden color.
Gradually whisk in the milk to form a smooth sauce. Continue cooking and stirring for 3-4 minutes, until the sauce thickens.
Add the cheddar cheese and parmesan cheese to the sauce, stirring until fully melted and smooth. Season with mustard powder (if using), garlic powder, salt, and pepper.
Place the steamed cauliflower florets into the prepared baking dish. Pour the cheese sauce over the cauliflower, ensuring it is well-coated.
Optional: Sprinkle breadcrumbs on top for a crunchy finish.
Bake in the preheated oven for 20-25 minutes, or until the top is golden and bubbling.
Garnish with fresh parsley if desired, and serve hot. Enjoy your delicious Baked Cauliflower Cheese!