Preheat the oven to 350 degrees.
Add a gallon of water into a large pot. Add two heavy pinches of kosher salt. Bring to a boil.
When the water comes to a boil, add the pasta and cook until it is almost al dente, but still a little bit chewy, so it will be perfect when it comes out of the oven. Drain the pasta in a colander and place into your cast iron skillet. In a large saucepan, melt the butter over low heat. When the butter is completely melted whisk in the flour, black pepper, mustard powder, and a pinch of kosher salt.
When the flour is fully incorporated, pour in the milk. Turn your burner to medium, stir often, until the mixture comes to a boil.
Let the mixture boil for 1 minute, stirring constantly. Now slowly pour in the shredded cheddar and whisk to combine. Pour the sauce over the pasta, mix to combine.
Now place the entire thing into your oven and bake for 25 minutes or until the top of the mac & cheese is beginning to brown. Be careful not to overcook it or the top layer will be too dry. Let cool for at least 5 minutes before enjoying.