Pull meat from chicken and refrigerate until needed.

Add 2 quarts of water to a pot and transfer in chicken bones, skin, and any meat scraps. Add bay leaf and bring to a simmer on high heat. Reduce to low and simmer for 2 hours. Remove to cool, about 1 hour. Strain broth and reserve in refrigerator until needed.

To make the pot pie mixture, add butter to a pot and melt over medium-high heat. Add onions, carrots, celery, and salt. Cook, stirring, until onions start to turn translucent, about 5 minutes.

Season with pepper, thyme, garlic powder, and cayenne. Cook for 1 minute more. Add flour and cook until flour toasts to a golden brown on the bottom of the pot, 3 to 4 minutes.

Stir in 6 cups of broth, and bring to a simmer on high heat, then reduce to medium. Add peas and cream and cook, stirring, for 10 minutes. Cut chicken meat into small bite-size pieces and add it in. Cook 5 minutes more. Turn off heat and reserve.

Preheat the oven to 375 degrees F (190 degrees C).

Fill a large pot with lightly salted water and bring to a rolling boil. Stir in pasta and return to a boil. Cook pasta uncovered, stirring occasionally, 1 minute less than specified in package directions. Drain very well; stir pasta into pot pie mixture. Transfer everything into a large, deep baking dish and distribute evenly.

Stir panko, salt, and melted butter together in a bowl; sprinkle evenly over casserole.