Preheat oven to 450 degrees.
Cook cauliflower in a pot of boiling salted water until just tender, 6 to 8 minutes. Drain cauliflower well in a colander and transfer to a 1-quart baking dish coated with non-stick cooking spray.
While cauliflower is cooking, melt 1 tablespoon butter in a heavy saucepan over moderately low heat and whisk in flour. Cook roux over low heat, whisking, 3 minutes. Add milk in a slow stream, whisking, and bring to a boil, whisking frequently. Reduce heat and simmer sauce, whisking occasionally, 8 minutes. Remove from heat and add cheese, salt and pepper, whisking until cheese is melted. Pour cheese sauce over cauliflower and stir gently to combine.
Add breadcrumbs to a small bowl. Melt remaining tablespoon butter; pour over breadcrumbs and toss to coat. Sprinkle crumb topping evenly over cauliflower.
Bake gratin in middle of oven until topping is golden brown, about 10 minutes.