Heat oven to 375°F. Gently simmer chicken, poaching water, and chicken salt in saucepan; cover and cook 20 to 25 minutes to at least 165°F. Drain; shred.
Meanwhile, boil shells with tablespoon salt to 3 minutes short of package time; drain. Melt 3 tablespoons butter in saucepan; whisk flour 1 minute.
Gradually whisk in milk; gently simmer 3 to 4 minutes until lightly thickened. Remove from heat; add cheddar, hot sauce, garlic powder, and pepper.
Fold in pasta and chicken; transfer to deep 9-by-13-inch dish. Foil-cover; bake 25 minutes.
Melt remaining butter; mix with panko. Uncover; add blue cheese and crumbs. Bake 8 to 10 minutes until golden and center 165°F.
Rest 10 minutes before serving.