Pour water into Instant Pot liner.
Add butter, salt, and mustard to water and stir to dissolve the salt and the mustard powder. (Butter will remain in its solid form.)
Pour elbow macaroni into the water mixture. DO NOT STIR! Simply tap down pasta gently to make sure that it is submerged under the water.
Put the lid on the Instant Pot. Lock it into place, and set the steam valve to "Sealing". Press the "Pressure Cook" or "Manual" button on the Instant Pot panel. Next, set the pressure time for 4 minutes, and set the pressure level to "High".
One the Instant Pot is finished cooking the macaroni, DO NOT QUICK RELEASE THE PRESSURE. Allow the pressure to come down naturally. This will allow the macaroni to finish cooking to the perfect "al dente" stage and completely absorb all the water. Plus, this will prevent the "foamy" steam from sputtering out the pressure release valve creating a mess.
Once the pressure has released naturally, remove the lid from the Instant Pot. The Instant Pot should have switched to the "Keep Warm" stage. If not, press the "Keep Warm" button on the front panel. DO NOT USE THE SAUTE OPTION. This will cause the pasta to stick to the bottom of the Instant Pot liner and potentially burn.
With the Instant Pot on "Keep Warm", begin to add a small handful of each cheese with a few ounces of the cream to the macaroni. Stir and allow the cheese to melt before adding additional cheese and cream. Once all the cheese and cream have been incorporated, the mac and cheese is ready to serve.
Optional: Once all the cheese and cream have been incorporated, and you would like to add some cream cheese for a bit of additional "creaminess", start by adding two ounces of cream cheese, working up to 4 ounces of cream cheese, until you achieve the desired consistency.