Add water, salt, butter and pasta to the instant pot.
Seal the lid and select manual pressure.
Set the timer for 4 minutes.
It takes 7-10 minutes before the timer begins to countdown.
After the beep of the timer, use the quick release valve to release pressure.
Open the instant pot and drain almost all of the remaining water from the noodles. You can leave a little.
Place the pot back in the pressure cooker,add mustard powder, pepper, milk and both cheeses.
Stir the cheeses into the noodles and replace the lid.
Allow everything to melt for about 5 minutes and stir again.
Store leftovers in an airtight container in the fridge.