In a large bowl, combine the chicken, 3/4 teaspoon salt, 1/2 teaspoon pepper, garlic powder and onion powder until evenly combined.
Set a large skillet over medium-high heat along with 2 tablespoons butter. Once melted, add in the chicken and cook, stirring often, until browned, about 5 to 8 minutes.
Add another 2 tablespoons of butter and stir in the onion, garlic, carrots and celery, cooking until softened and just beginning to brown, about 3 minutes. Season with the remaining salt and pepper and set the pan off to the side to cool slightly.
Preheat oven to 375 degrees F. Grease a 9x13-inch casserole dish and set aside.
In a large bowl, stir together the cream cheese, 1/2 cup buffalo sauce, ranch, and scallions until smooth and evenly combined. Pour into the pan with the chicken and stir in the egg noodles, 1/2 cup Cheddar and 1/2 cup Monterey Jack. Mix everything until evenly incorporated.
Pour into the prepared baking dish and drizzle with the remaining buffalo sauce. Top with the remaining cheeses and bread crumbs. Melt the remaining 2 tablespoons butter and drizzle over the top.
Bake until golden brown and bubbly, about 18 to 20 minutes. Remove from the oven and top with sliced scallions before serving.