Cook macaroni according to package directions, drain and set aside.
In a medium-sized sauce pan, heat cream to a simmer and reduce heat to low. Stir 1 C of each cheese into the cream followed by the mustard and salt, stirring until cheese is fully melted. Remove from the heat and let rest for 2-3 minutes. You'll notice that the sauce will be thick and almost rubbery. This is the proteins in the cheese tightening up so just give it a rest for a few minutes to loosen things up and it will become creamy again.
After resting, season to taste with pepper and stir in cooked macaroni and remaining ¼ C of each cheese. Adding the cheese at the end will give it that little extra "cheese pull" with each bite and makes it so much fun to eat!