Add the water, half the butter, and salt to the Instant Pot. Heat the mixture using the SAUTE function on NORMAL.
When boiling, add the pasta, press CANCEL, put the lid on, PRESSURE COOK on HIGH for 4 minutes.
Allow for a 2 minute natural release, then quick release. Be careful as the butter may come out the valve.
Add the cheese, half & half, cayenne, and remaining butter. Once stirred in, add the lid with the valve open and let sit for 4-5 minutes.