Cook chicken breasts in boiling salted water for 15-18 minutes until 165°F. Cool and shred.
Steam broccoli until tender-crisp, 5-6 minutes. Drain well and pat dry.
Preheat oven to 350°F. Grease 9x13-inch baking dish.
Melt 4 tablespoons butter in saucepan. Whisk in flour, cook 1-2 minutes.
Gradually add milk while whisking. Cook 4-5 minutes until thickened.
Add cream cheese, cheddar, garlic salt, pepper, and onion powder. Whisk until smooth.
Spread broccoli in baking dish. Top with shredded chicken.
Pour cream sauce over chicken and broccoli.
Mix crushed crackers, melted butter, and Parmesan. Sprinkle over casserole.
Bake 30-35 minutes until bubbling and golden brown.
Let rest 10 minutes before serving.