Preheat the oven to 350 degrees F and butter a 13-by-9-inch baking dish.
Bring a large pot of salted water to a boil and fill a large bowl with ice water. Cook the cauliflower until just tender, about 5 minutes. Use a slotted spoon or spider to remove the cauliflower and immediately submerge it in the ice water to cool completely, then drain. Bring the salted water back to a boil and cook the pasta until al dente, about 7 minutes. Drain and set aside.
Meanwhile, melt 3 tablespoons butter in a large skillet over medium heat. Add the celery and onion and cook until soft, about 5 minutes. Stir in the blanched cauliflower and garlic and cook 2 minutes, then add 1/2 cup hot sauce and simmer until slightly thickened, about 1 minute more.
Melt 2 tablespoons butter in a medium saucepan over medium heat. Whisk in the flour and mustard until smooth. Whisk in the half-and-half, then add the remaining 1/4 cup hot sauce and stir until thick, about 2 minutes. Whisk in the Cheddar and pepper jack cheeses, then whisk in the sour cream until smooth.
Spread half of the macaroni in the prepared baking dish and pour over about half of the cheese sauce. Top with the cauliflower mixture and the remaining macaroni. Pour the remaining cheese sauce evenly on top.
Put the remaining 2 tablespoons butter in a medium microwave-safe bowl and microwave until melted, about 30 seconds. Stir in the panko, blue cheese and parsley. Sprinkle over the macaroni and bake until bubbly, 30 to 40 minutes. Let cool 10 minutes, then serve with additional hot sauce on the side.
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