Preheat your oven to 350 degrees. Butter the bottom and sides of a 9'x13" baking dish with the butter.
Place the chicken breasts in a medium-sized pot and cover with water. Bring to a boil over high heat then reduce the heat and simmer for 12 minutes, or until the chicken is fully cooked and no longer pink in the middle. Remove the chicken from the pot and shred it using 2 forks. (Or chop the chicken, if you prefer)
While the chicken is cooking, make the sauce. Melt the butter in a medium-sized pan over medium heat. Add the flour and whisk for 1 minute. Add the remaining sauce ingredients and whisk until combined and the cheese has melted.
To make the topping, melt the butter in a small pan or microwave. Mix the panko, salt, and pepper into the melted butter.
Place the shredded chicken in the buttered casserole dish. Pour the sauce over the top and mix it into the chicken. Top with the grated cheddar cheese and the buttered breadcrumbs.
Cover the casserole with foil and bake for 30 minutes, or until hot and bubbly, removing the foil halfway through.