Add in the butter, cayenne, paprika, and dry mustard.
Cut the chicken breasts into bite-sized pieces and add to the Instant Pot.
Layer the pasta on top of the other ingredients.
Pour in the chicken broth and gently submerge the pasta under the broth.
Close the lid and set to MANUAL/PRESSURE COOK for 5 minutes.
After pressure cooking, carefully and slowly release pressure by opening and closing the vent until all the pressure is released. (Do this slowly to prevent liquid from spewing from the vent.)
Remove the lid, set to SAUTE, and add in the cheddar cheese.
Mix together the milk and corn starch and add to the pasta.
Stir and allow the sauce to thicken.
Serve with Parmesan cheese and fresh parsley if desired.