• Steam the brocolli until tender-crisp and set aside.

To make the white cheddar sauce:

• Melt the butter in a medium saucepan over medium heat. Add 1 tablespoon of flour at a time, whisking well after each addition. Cook for 2-3 minutes, being careful not to brown the roux.
• Add the warm milk slowly, whisking constantly. When all the milk has been added bring to a boil (keep whisking). Immediately remove from heat, season with salt and pepper.
• Stir in the heavy cream, then add 1 cup of the white cheddar cheese, whisking to incorporate. Add the remaining cup of cheddar cheese, again whisking until sauce is smooth.

To assemble:

• Butter the bottom of a 9x13 inch baking dish with half the butter. Spread the cooked pasta over the bottom of the dish then layer the steamed broccoli florets over the pasta. Lastly add the cooked shredded chicken over the broccoli. Pour the white cheddar sauce, as evenly as possible over the top of the casserole, shifting contents to remove an large air pokets.

Make the topping:

• Melt 1 tablespoon butter in a sauté pan. Remove from heat and add the panko bread crumbs. Stir to coast crumbs with butter. Top the casserole evenly with the crumb topping.
• Bake casserole uncovered at 375 degrees for 30 minutes or until top is golden brown.