In a small bowl, mix together cayenne, salt, black pepper, and garlic powder together.
Season chicken breasts on both sides spice mixture.
Press the sauté button on the Instant Pot and wait for it to say "hot." Add olive oil and place chicken breasts in Instant Pot. Cook for about 2 minutes before flipping and cooking an additional 2 minutes on the other side. Remove from pot to separate plate.
Add onion and garlic to Instant Pot and let cook for one minute.
Pour half and half and ½ cup chicken broth into pot and bring to a boil. Deglaze by scraping the brown bits off the bottom of the pot.
Break spaghetti in half and place in Instant Pot. Try to scatter the spaghetti in different directions so it won't clump up when cooking. Top pasta with chicken breasts.
Pour in remaining 1 ½ cups chicken broth and buffalo sauce.
Press "Manual" on Instant Pot and set for 7 minutes. When time is reached, let Instant Pot naturally release pressure for 8 minutes, before manually releasing it the rest of the way. Open pot.
Remove chicken breasts to cutting board; slice. The pasta may look very liquid-y, but if you give it a couple stirs, the sauce will completely coat spaghetti.
Serve pasta on individual plates or bowls and top with sliced chicken. Sprinkle crumbled blue cheese and chopped parsley over the top.