Pound chicken breasts to even thickness and season with salt and pepper.
Dredge chicken in flour, shaking off excess.
Heat 2 tablespoons of olive oil in a large skillet over medium-high heat. Cook chicken until golden brown and cooked through, about 5 minutes per side. Remove from skillet and set aside.
In the same skillet, heat remaining 2 tablespoons of olive oil. Add mushrooms and garlic, sauté until mushrooms are golden brown and garlic is fragrant.
Pour in Marsala wine and chicken broth, scraping up any browned bits from the bottom of the skillet. Simmer until liquid is reduced by half.
Stir in butter until melted and sauce is slightly thickened.
Return chicken to skillet, spoon sauce over the chicken, and simmer for another 2-3 minutes until heated through.
Garnish with chopped parsley before serving.