Pound out chicken breasts to 1/2" thickness.
Combine flour, salt, pepper & garlic powder on a plate. Coat chicken breasts in flour mixture.
Heat 2 T olive oil in a large skillet over medium heat. Add chicken to pan, cook chicken until each side is browned, 3-4 minutes per side. Remove chicken from pan.
Add 1 T olive oil and 1/2 T butter to pan. Sauté mushrooms in pan, season with salt & pepper, and cook until softened and browned.
Turn heat up to medium-high, add Marsala and deglaze the pan, using a wooden spoon to scrape up any browned bits off the pan bottom.
Add chicken back to pan to warm through. Let wine reduce by half, then swirl in 1 T butter to thicken the sauce. Garnish with parsley and serve immediately.