Preheat the oven to 375°F with the oven rack in the lower third of the oven. Lightly grease a 13-by-9-inch baking dish and set aside.
Boil the pasta shells in generously salted water according to the package directions (about 10 minutes). Drain and remove the shells to a tray or baking sheet so they are spread out and do not stick to each other. Allow them to cool.
Meanwhile, heat the olive oil over medium-high heat in a skillet and add the celery, cooking for about 5 minutes, stirring occasionally until crisp-tender. Remove from the heat and add to a medium mixing bowl along with the shredded chicken breast, ricotta cheese, ¼ cup of the mozzarella, half the hot sauce (⅓ cup), the egg, and the crumbled blue cheese, if using.
Spoon the chicken mixture into the cooked pasta shells dividing the mixture evenly between 20 shells. You'll have a couple extra cooked shells in case any of them rip, or if you have a little extra filling.
Place the shells into the greased baking dish and drizzle the tops of the shells with the blue cheese dressing and the remaining ⅓ cup hot sauce. Top the shells with the remaining ½ cup shredded mozzarella cheese.
Cover the baking dish with foil and bake for 15 minutes, then uncover and continue to bake another 10 to 15 minutes or until bubbly and heated through. Top with sliced scallions to garnish, and serve immediately.
Notes
This nutritional information was calculated with store-bought blue cheese dressing, however I like to make my own with the following measurements. I'm able to reduce the calories/fat significantly by using light mayonnaise and sour cream when I make my own. Make the dressing a day or so in advance to give it time for the flavors to develop. Refrigerate until needed.
To make ¾ cup homemade blue cheese dressing combine ¼ cup plus 2 tablespoons sour cream, 3 tablespoons mayonnaise, 3 tablespoons buttermilk or milk, 3 tablespoons crumbled blue cheese, ½ tablespoon red wine vinegar and ½ teaspoon kosher salt (or more to taste).
If you want these Buffalo chicken stuffed shells to be extra spicy, increase the hot sauce to ¾ cup, and add ½ cup to the filling and drizzle the remainder over the top.
If you're allergic to eggs, you could omit the egg in the filling, but the result will be a bit of a looser filling.