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