Bring a pot of water to boil. Add salt and the pasta, and cook until just shy of al dente, about half as long as the box suggests (it will continue cooking in the oven). Drain and set aside.
In a small saucepan, heat the hot sauce and 2 tablespoons butter, and swirl together to incorporate. Reserve ¼ cup of the buffalo sauce and set aside. Add the remaining sauce to the shredded chicken and mix well. Set aside until needed.
Preheat the oven to 400°F. Over medium-high heat, melt the remaining 4 tablespoons butter in the same pot used to boil the pasta. Add the flour to make a roux and whisk until it is nice and smooth. Cook for a couple minutes to cook out the raw flour flavor, but do not allow the roux to brown. Add the hot milk a little at a time, whisking in between each addition, until all of it has been added. Cook, constantly whisking, about 7 to 10 more minutes on medium heat until thickened, allowing it to bubble and simmer, but not come up to a full boil. Season with salt and pepper. Remove from the heat and whisk in the shredded cheeses, a little at a time until completely combined.
Once the cheeses are all combined and melted, add the pasta and stir to coat. It will be very saucy. Fold in the celery and shredded buffalo chicken. Pour the pasta mixture into a greased 3-quart shallow baking dish. Drizzle ½ of the reserved buffalo sauce over the top.
Add ½ cup of the panko to a small mixing bowl and add the garlic and blue cheese. Use your fingers to cut the blue cheese chunks into the panko as if you’re making biscuits, yielding a mixture of pea-sized blue cheese/panko chunks. Add the remaining buffalo sauce and combine. Crumble the mixture over the top of the casserole.
In the same small mixing bowl, add the remaining panko and olive oil and mix to combine. Distribute the panko mixture over the pasta. You now have a double crust of blue cheese-buffalo-panko crumbs, and regular panko crumbs. Bake for 20 to 25 minutes until nice and bubbly, and golden brown. Allow the pasta to rest for about 10 to 15 minutes before serving, as it will be extremely hot and also need a few minutes to set.