A different take on Instant Pot mac & cheese that does not involve any pressure cooking.
Add the dry pasta to the Instant Pot. Cover with just enough water to cover the pasta. Add kosher salt so that the water taste a little salty.
Hit the saute button.
Stir the pasta frequently as it comes to a boil.
Once the pasta is just short of being al dente, add in the evaporated milk and ground mustard powder.
Keep stirring and bring the milk to a simmer.
After you reach a simmer, turn off the Instant Pot, and add in 1/3 of the cheese. Stir until melted.
Add next the 1/3 and stir until melted. Add in the rest of the cheese and keep stirring until it thickens up. If will look soupy at first but if you keep stirring it will eventually thicken up. It make take a few minutes.
Serve immediately or set the Instant Pot to keep warm for later service.
If you don't have an Instant Pot, just make this in a regular pot either on the stove, on a hot plate, or induction burner. Heat up over high heat and reduce to medium low once you hit a boil. Once the milk is added make sure you only get it up to a simmer, do not boil.