Grate the cheddar and parmesan cheeses and set aside. (It's better to grate these cheeses fresh as store-bought pre-grated cheese is coated in either cornflour or cellulose, which will make your mac and cheese thick and gloopy.)
Pour the macaroni pasta into the Instant Pot.Add the chicken broth and water (do not stir as this will also make your macand cheese gloopy and thick due to starches being released).
Spoon in the required amount of salt.
Close the lid and set the instant pot to pressure cook on high for 4 minutes.
After 4 minutes, do a quick-release on the Instant Pot and immediately open the lid (or it will continue cooking).
Turn the Instant Pot on low, sauté mode for 1 minute and add all of the cheeses, heavy cream, butter, and black pepper. Stir until all ingredients are incorporated.