Turn the instant pot setting to sauté. Add in the the olive oil, chicken, basil, salt and pepper, and sauté about 3 minutes on each side. Remove the chicken and set aside. Add the butter to the pot and sauté the mushrooms.
Allow them to get some good color, then remove and set aside. Add in the garlic and sauté for 20 seconds, careful not to burn it! Deglaze the pot with the marsala wine, scraping up the brown bits on the bottom.
Stir in the chicken stock and most of the parsley (reserving some for garnish), then add the chicken and mushrooms back in. Turn off the instant pot and set to pressure cook for 10 minutes, then instant release the pressure.
Remove the chicken and mushrooms, then set the instant pot back to the sauté setting . Stir in the mascarpone cheese, heavy cream and Xanthan Gum, then add the chicken back in and cook an additional 5-8 minutes or until thickened. Serve with your favorite veggie or zucchini noodles and enjoy!