Preheat oven to 375. Coat a 2 quart baking dish with unsalted butter. Blanch broccoli in large pot of boiling salted water til tender, about 3 minutes; drain.
Melt 4 Tbsp. butter in large pan over medium high heat. Whisk in flour; cook til browned and fragrant, 2 minutes. Slowly whisk in milk. Stir in cheddar, 1/2 cup Parmesan, Dijon, paprika and garlic powder
Cook sauce til thickened, 4 to 5 minutes. Season with salt and pepper. Stir in chicken and broccoli; transfer mixture to prepared dish. Top casserole with panko, almonds, and remaining 1/4 cup Parmesan. Melt remaining 2 Tbsp. butter and pour over top of casserole.
Bake casserole til topping is golden, 25 to 30 minutes. Serve casserole with hot rice or buttered egg noodles.