Preheat your oven to 375°F. Grease a large cast iron skillet with butter.
In a large pot, bring heavily salted water to a boil. Cook the elbow macaroni according to the package instructions until al dente. Cook one minute less than package instructions. Drain and set aside.
In the same pot, melt the butter over medium heat. Add the evaporated milk, heavy cream, dry mustard, salt, pepper, cayenne and optional annatto powder. Stir well to combine and let the mixture simmer for a few minutes. Remove from heat.
Add 1.75 cup of extra sharp cheddar, 1.25 cup of medium sharp cheddar and the Parmesan cheese to the pot with the milk mixture. Stir until the cheeses are melted and the sauce is smooth.
Add the cooked macaroni to the pot and stir until well coated with the cheesy sauce.
Transfer the mac and cheese mixture to the prepared cast iron skillet, spreading it out evenly.
Sprinkle the remaining 1/4 cup of extra sharp cheddar and 1/4 cup of medium sharp cheddar over the mac and cheese.
In a medium bowl, combine the bread cubes, melted butter, minced garlic, salt and pepper for the garlic butter croutons. Toss until the bread cubes are well coated.
Sprinkle the garlic butter croutons evenly over the top of the shressed cheese.
Bake for 20-25 minutes, until the mac and cheese is bubbly.
Switch the oven to broil and broil for an additional 2-3 minutes until the croutons are crispy and golden brown.
Remove from the oven and let it cool for a few minutes before serving.