Step 1

Preheat oven to 375°F. Coat a 13- x 9-inch baking dish with cooking spray. Prepare pasta in a large stockpot according to package directions for al dente. Drain pasta; return to stockpot, and set aside.

Step 2

While pasta cooks, melt 2 tablespoons of the butter in a large saucepan over medium. Add flour; cook, whisking constantly, until golden brown, about 1 minute. Gradually add half-and-half, whisking constantly; cook, whisking often, until mixture is thickened and coats back of a spoon, about 3 minutes. Reduce heat to low; gradually add 2 cups of the Cheddar 1/2 cup at a time, stirring until melted and smooth after each addition. Remove from heat; whisk in hot sauce, sour cream, Worcestershire, and salt. Pour sauce over drained pasta in stockpot; add chicken, carrots, and celery, and toss gently to evenly combine. Spoon mixture into prepared baking dish.

Step 3

Melt remaining 2 tablespoons butter in a small saucepan over low. Stir together melted butter and panko in a small bowl. Sprinkle casserole evenly with remaining 1 cup Cheddar and panko mixture. Bake in preheated oven until casserole is golden brown and bubbly, 25 to 30 minutes. Sprinkle evenly with blue cheese, and garnish with celery leaves. Serve hot.