Preheat Oven : Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish.

Cook Pasta and Broccoli : Bring a large pot of water to a boil. Once boiling, add your pasta and cook until it's just tender. In the final couple of minutes, toss in the broccoli florets and cook until bright green and tender-crisp. Drain both the pasta and broccoli, and set them aside.

Make the Creamy Base: In a medium-sized saucepan, melt butter over medium heat. Once melted, add the flour and cook, stirring frequently, for about a minute until it forms a smooth paste. Slowly pour in the milk while whisking to avoid lumps. Continue stirring until the sauce thickens and becomes velvety. Season with garlic powder, salt, and pepper to taste.

Combine Ingredients : In a large mixing bowl, combine cooked pasta, broccoli, chicken, and the prepared sauce. Mix well to coat evenly. Stir in half of the cheddar cheese.

Assemble the Bake : Transfer the mixture to the prepared baking dish. Top with the remaining cheddar cheese and grated Parmesan.

Cook in the Oven: Place the dish in the preheated oven and let it cook for 20 to 25 minutes, or until the cheese is fully melted and starts to bubble. For a crispy, golden crust on top, switch the oven to broil and cook for another 2 to 3 minutes, watching closely to prevent burning.

Serve : Remove from the oven and let cool for 5 minutes. Garnish with fresh parsley before serving.