Gradually whisk in the half-and-half until the sauce is smooth and thickened. Stir in the cream cheese and Parmesan until melted and incorporated. Stir in the garlic powder, salt, and pepper, then remove from heat.
2½ cups half-and-half, 4 ounces cream cheese, ½ cup freshly shredded Parmesan cheese, 1 teaspoon garlic powder, 1 teaspoon kosher salt, ¼ teaspoon ground black pepper
Combine the pasta, broccoli, chicken, and sauce in a large bowl and stir to combine. Transfer mixture to prepared baking dish. Sprinkle with mozzarella.
Bake for 15 minutes, then broil for 2-3 minutes on high until the cheese is bubbly and browned. Serve warm.
Video
Notes
It's best to steam fresh, not frozen broccoli. Freshly steamed broccoli will hold its shape better than frozen.
Don't forget to broil the casserole for just a few minutes to get that wonderfully browned mozzarella top! Keep your eye on it as it broils though, because it can quickly go from golden browned to burned.
I use unsalted butter, but salted works as well. However, if you use salted butter, I recommend omitting the salt that's called for from the recipe.
Storage: Store baked chicken broccoli alfredo in an airtight container in the refrigerator for up to 3 days, or in the freezer for up to 3 months.