Prepare the JSF Buffalo-style chicken according to package instructions. Let cool slightly, then cut into bite size pieces.
Meanwhile, in a medium skillet, melt ¼ cup (4 tablespoons) of the butter. Add the panko and ranch seasoning and cook over moderately-low heat, stirring frequently, until the breadcrumbs are golden brown, about 5 minutes. Season with salt. Scrape into a small bowl and set aside.
Preheat the oven to 350°F and grease a 9-by-13-inch baking dish.
In a large pot of salted boiling water, cook the pasta until just al dente (it will continue to cook in the oven). Drain well.
In a large pot, melt the remaining ¼ cup (4 tablespoons) of butter. Sprinkle the flour over the butter and cook over moderate heat, whisking constantly, until the flour smells toasty, about 1 minute. While whisking, stream in the milk. Bring to a boil over moderate heat, whisking frequently, until the bechamel is thick enough to coat the back of a spoon, 3 to 5 minutes. Remove from the heat and stir in the 4 cups of shredded cheddar cheese and the hot sauce. Season with salt.
Let the cheese sauce cool slightly, then fold in the cooked pasta, Buffalo chicken, blue cheese and the cubed cheddar. Season with salt and more hot sauce, if desired. Scrape the mac and cheese into the prepared baking dish and sprinkle the ranch breadcumbs generously over the top.
Set the casserole on a rimmed baking sheet and bake for about 25 minutes, or until the sauce is bubbling and the top is golden brown and crispy. Transfer to a rack and let cool for 10 minutes. Garnish with celery leaves and serve, passing more hot sauce at the table.