Preheat oven to 350f
Butter or oil casserole dish
Prepare macaroni. Follow box time, but slightly shorter than for al dente, as macaroni will cook during the baking. Set aside.
Prepare roux. Melt butter over low heat, until lightly simmering. Add flour and allow to cook to light brown, whisking as needed. Add remaining dry ingredients for another minute or two.
Bring milk(s) to room temp, or slightly warmer. Slowly add to roux, whisking constantly to remove lumping, until thickened to syrup consistency.
Gradually add 3 cups cheese, whisking to smooth consistency. Once cheese & roux are mixed nicely, combine macaroni with sauce, and transfer to casserole dish.
Top with remaining cheese.
Sprinkle bread crumbs evenly across cheese topping.
Lastly, add butter dots evenly across bread crumbs.
Bake for 30 minutes, or until bubbly top and lightly browned. If stored frozen, bake 45 minutes to an hour.