Preheat the oven to 375℉
Cook the pasta for about 7-8 minutes (I prefer al dente), set aside
In the same pot, melt the butter, once it is melted, add in the flour to make a roux
Slowly stir in the warm milk and add the salt, pepper, mustard, and red pepper flakes
Continue stirring until sauce has thickened, a few minutes
Take sauce off the heat and stir 1 ¾ cups sharp cheddar until it's just about melted, then add in the pasta and stir to combine
In a medium size baking dish, pour in the mac and cheese and top with remaining ¼ sharp cheddar and sprinkle with breadcrumbs
Bake for approx. 40 minutes - serve hot!