Place 2 large boneless, skinless chicken breasts on cutting board or plate. Season each chicken breast with salt and freshly ground black pepper to taste, then flip chicken breasts over and repeat on other side. Set chicken aside.
Set Instant Pot to Sauté mode and add 2 tablespoons unsalted butter to Instant Pot insert. Let butter melt completely.
When butter has melted, place seasoned chicken breasts in Instant Pot. Sear chicken 2 minutes, then flip chicken breasts over. Sear chicken 2 minutes more or until opaque on both sides.
Once chicken is seared on both sides, transfer chicken breasts to large plate and set aside. Turn off Sauté mode.
Add 3 cups chicken broth, 1 cup whipping cream (at room temperature), 1 teaspoon garlic powder, ¼ teaspoon freshly ground black pepper, and salt to Instant Pot insert. Stir to incorporate ingredients, making sure to scrape up any bits of chicken that may have stuck to bottom of insert.
Break 1 pound fettuccine in half, then lay pasta on top of sauce mixture. Do not stir pasta into sauce.
Place seared chicken breasts on top of pasta. Do not stir chicken or pasta into sauce.
Secure lid on Instant Pot with valve in Sealing position. Set Instant Pot to Manual High Pressure, then set cook time to 7 minutes. Instant Pot will pressurize and then 7-minute cook time will begin.
After 7-minute cook time ends, carefully Quick Release internal pressure. Use caution as any escaping steam will be very hot. Once pressure has released, carefully remove lid from Instant Pot and set aside.
Transfer chicken breasts to clean cutting board. Set chicken aside to rest 2 to 3 minutes.
While chicken rests, set Instant Pot to Sauté mode. Add 4 ounces cream cheese and 1 cup finely shredded fresh parmesan to Instant Pot and whisk ingredients together until fully combined and melted.
Slice chicken breasts into thin strips, then return chicken to Instant Pot. Stir chicken into sauce and pasta. Note: if sauce is thinner than desired, simmer mixture on Sauté mode 2 to 3 minutes, stirring often.
Portion chicken alfredo onto serving plates. Garnish as desired and serve warm.