Turn Instant Pot to saute function and add butter to melt.
Dredge chicken breasts in flour on both sides, then place into Instant Pot. Let them brown in the butter for 1-2 minutes per side, then remove and set aside.
Add garlic, pancetta, mushrooms and marsala wine and saute for 2 minutes.
Turn Instant Pot off. Add chicken breasts back in, along with chicken broth, nestling chicken down into the mushrooms.
Using the manual (or pressure cook) button, set the timer for 8 minutes.
Use quick release and remove just the chicken breasts from the pot.
Switch the Instant Pot to saute and wait 1 minute until sauce starts to bubble. Add the mixture of cornstarch and water and let simmer 5-10 minutes, stirring occasionally while sauce thickens.
Place chicken on plates, spoon mushroom sauce over chicken, garnish with parsley and serve warm.