Prepare oven: Preheat oven to 350°F. Spray an 8-inch square baking dish with cooking spray.
Cook cauliflower: Trim cauliflower into florets. Bring a large saucepan of salted water to a boil over high; add cauliflower, and boil until just tender, 4 to 5 minutes. Drain cauliflower, and return to saucepan.
Mix casserole filling: Stir together cream of chicken soup and sour cream in a small bowl; add to cauliflower, and toss to combine.
Move filling to baking dish: Transfer mixture to baking dish. Sprinkle casserole with cheese and panko; dot with butter slices.
Bake casserole: Bake in preheated oven until casserole is bubbly and panko is golden, about 30 minutes. If desired, top with parsley.
