Bring a large pot of water to a boil. Add 1 tablespoon of Kosher salt.
Once boiling, add the dry pasta and cook until al dente (check the package for timing).
While the pasta cooks, melt the butter in a large pot over medium heat. Add the flour and whisk until smooth. Cook for 1 minute to remove the raw flour taste, then stir in the garlic powder, 1/4 teaspoon Kosher salt and the black pepper.
Slowly whisk in the milk in three additions, letting the sauce thicken before adding each addition.
Once all the milk is in, raise the heat to medium-high. Bring the sauce to a gentle simmer and let it bubble for 30 seconds or until it thickens to the consistency of heavy cream. Turn off the heat immediately once the sauce has thickened.
With the heat off, add the shredded cheddar cheese and stir until smooth, creamy, and completely melted. If the sauce is too thick, add a little milk to thin it out.
When the pasta is done boiling, drain it and then immediately add it to the cheese sauce. Gently mix until the shells are thoroughly coated. Give the mac and cheese a taste, and add more salt and pepper if needed. Serve hot.