Preheat the oven to 350˚F, and butter an 8" X 8" baking pan.
Cook the pasta 2 minutes less than on the package directions in well salted water. It should be barely al dente. Remove the pasta to a large bowl and add the cauliflower to the boiling water. Cook until just tender, about 7 minutes. Drain, retaining 1/2 cup of the pasta-cauliflower water, and add to the bowl of pasta.
Melt 1 1/2 Tbsp of the butter in a medium heavy pot over medium heat. Whisk in the flour, and cook for 1 minute, continuing to whisk. Gradually whisk in the milk and continue to whisk until thickened, about 2 - 3 minutes, and gradually whisk in the water. When it thickens again, whisk in the grated cheese, salt and pepper. Lower the temperature to low.
Whisk together the Bang Bang Sauce and add to the cheese. Add 2 cups of the Bang Bang cheese sauce to the pasta and cauliflower and toss. Add more as desired. I used 2 1/2 cups, but the recipe should make about 3 cups.
Combine the Parmesan and breadcrumbs in a bowl. Melt the remaining butter, mix with Parmesan and breadcrumbs, and scatter over the casserole.
Bake for 15 - 20 minutes, or until the casserole is bubbly, and the top is lightly browned.
Scatter a little chopped parsley over the top and serve immediately.