Horizontally slice each chicken breast right through the centre to create 4 even-sized breasts.
In a large shallow dish combine the flour, salt, garlic powder and black pepper. Dredge each chicken breast through the flour, then shake off excess and place to one side.
Add 1 tbsp oil and butter to a large pan over medium-high heat. Add the chicken and fry both sides for a few mins, or until golden on the outside and just about cooked through the centre. Remove them from the pan and place them to one side.
Lower the heat slightly to medium, then melt in 2 tbsp butter (save the final tbsp for later). Add the shallots and fry for a couple of mins until they soften, but aren't quite yet turning golden. Add the garlic and fry for 30 seconds or so, then add in the mushrooms and fry until soft and golden.
Raise the heat to high and pour in the marsala wine. Simmer for 3 mins to reduce it by a little over half. Lower the heat back down to medium and stir in the stock and cream.
Simmer until the sauce begins to thicken, around 7-10 mins (it will thicken, just keep simmering and stirring away). Add the final tbsp of butter and swiftly stir until it melts and emulsifies with the sauce. Check for seasoning and adjust if needed.
Add the chicken back in the pan and baste in the sauce to coat it and warm it through. Garnish with parsley then serve up and enjoy!