Preheat the oven to 350 degrees F. Make the toasted panko by adding the butter over medium heat. Once melted, add the panko and a pinch of salt. Cook until toasted, stirring often so you don't burn the breadcrumb, until light-medium brown, about 5 minutes. Remove and set aside.
In the meantime, cook the macaroni 2 minutes shy of the recommended directions. Once done, drain and run under cold water. Return to the pot with one tbs of butter, stir, and set aside. (Note: If you're baking the mac and cheese in a baking dish instead of the cast iron, you can add it right in there now.)
To a cast iron skillet set of medium-low heat, add the butter. Once melted, add the flour and whisk until it forms a paste (this is your roux).
Now, lower the heat slightly and begin SLOWLY adding your milk while whisking. I like to had some melt, whisk until fully incorporated, and then add more milk. I continue this until all of the milk is added and there are no lump.
Next, add the spices and whisk to combine. Turn the heat up slightly and let cook for about 8 minutes, whisking to make sure the bottom doesn't burn or stick to the pan. You'll know the sauce is ready when it coats the back of a spoon and you can make a clean line with your finger. Remove the pan from the heat. Slowly begin adding the cheese a little at a time, stirring it in. When all of the cheese in added, pour it directly over the macaroni (whether in the pot or a baking dish). Stir to combine.
If baking in the cast iron, pour the macaroni and cheese back in where you cooked the cheese sauce. Add the toasted panko in an even layer.
Bake in the preheated oven for 18-20 minutes or until it begins to bubble. DO NOT over bake or you risk losing the sauce. Remove and serve.