Place butterflied chicken into a mixing bowl. Add in the onion powder, garlic powder, sea salt flakes, black pepper & olive oil. Rub the chicken into the seasoning & set it aside for the time being whilst you prepare the rest of the ingredients.
Place a large pan over high heat. Once hot, add in the olive oil & unsalted butter, allow to melt. Once melted, add in the seasoned butterflied chicken breasts & sear for 3 minutes on both sides until golden brown. Remove chicken from pan & place onto a plate to rest.
In the same pan, add in the butter if using then add in swiss brown mushrooms & season to taste. Saute for 3 minutes, stirring regularly. Add in minced garlic & saute for 45 seconds, stirring regularly.
Add in the marsala fortified wine as well as the chicken stock, stir well to deglaze the pan & bring to a boil. Once boiling, reduce the heat to medium-low & simmer for 7 minutes to reduce, thicken & enhance the flavour.
7 minutes later, add in the cream, check for seasoning, increase the heat to medium-high & bring the sauce to a simmer. Once simmering, reduce heat back to medium-low & allow the sauce to reduce for 4 minutes, stirring occasionally.
Add in chopped parsley & reintroduce the seared chicken along with any juices that may have accumulated on the plate or bowl. Flip the chicken in the sauce & cook for 1 minute. Remove from the stovetop serve & dig in.