Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish and set aside.

Blanch the broccoli: Bring a pot of salted water to a boil. Add the broccoli florets and cook for 2-3 minutes until bright green. Drain and set aside.

Make the cheese sauce: In a saucepan, melt 1/4 cup butter over medium heat. Whisk in the flour and cook for 1-2 minutes until golden. Gradually whisk in the milk until smooth.

Season the sauce: Stir in garlic powder, onion powder, salt, and black pepper. Cook until the sauce thickens, about 3-4 minutes. Remove from heat and stir in sour cream, cheddar cheese, and Parmesan cheese until melted.

Combine broccoli and sauce: Add the blanched broccoli to the cheese sauce and mix well. Transfer the mixture to the prepared baking dish.

Add the topping: In a small bowl, mix breadcrumbs with 2 tablespoons melted butter. Sprinkle evenly over the broccoli mixture.

Bake: Place the casserole in the preheated oven and bake for 25-30 minutes, or until the top is golden brown and bubbly.

Serve: Let the casserole cool for 5 minutes before serving.