Heat the heavy cream, cream cheese and garlic salt in a medium cast-iron skillet over high heat. As the edges start to bubble, gently whisk the cream cheese until it is evenly incorporated into the heavy cream, about 1 minute. Allow to cook for 15 seconds so the mixture thickens. Add the Cheddar and American cheese and continue whisking. Pour the sauce over the pasta, stir and serve immediately!
If you want a little color and crisp on the mac and cheese, put the mixture back in the skillet, add some breadcrumbs on top and place under the broiler for a couple of minutes. Feel free to substitute other leftover cooked pasta if you don't have cavatappi.