Pound the Chicken Breasts: Place one chicken breast inside a zip-lock bag or between two sheets of plastic wrap on a cutting board. Strike the thickest section of the breast with a meat mallet, working outward in even strokes from the center toward the edges, until the breast reaches a uniform 1cm / 0.4in thickness. Repeat with the remaining 3 breasts. Even thickness across all four cutlets prevents the thinner edges from drying out before the center reaches a safe internal temperature.
Dredge the Cutlets: Combine the flour, kosher salt, black pepper, and garlic powder in a shallow dredging dish and whisk until evenly blended. Press each pounded cutlet into the flour mixture, coating both sides completely, then shake off any excess flour over the dish. Lay the dredged cutlets in a single layer on a clean plate. A thin, even flour coating creates the fond that will later flavor the Marsala sauce.
Sear the Cutlets: Heat 2 tablespoons of olive oil in a 12-inch stainless steel skillet over medium-high heat for 2 minutes until the oil shimmers and a pinch of flour dropped into the pan sizzles immediately. Add two cutlets to the skillet without overlapping and sear for 4 minutes per side until the surface is deep golden brown and the internal temperature reads 74°C / 165°F on an instant-read thermometer. Transfer the cooked cutlets to a clean plate and tent loosely with foil. Repeat with the remaining tablespoon of oil and the two remaining cutlets.
Saute the Mushrooms: Reduce the heat to medium and add 1 tablespoon of butter to the same skillet, allowing it to melt into the remaining oil and fond. Add the sliced cremini mushrooms in a single layer and cook for 5 minutes without stirring until the mushrooms release their moisture and the cut edges turn golden brown. Add the minced garlic and stir continuously for 1 minute until fragrant and lightly golden.
Deglaze with Marsala Wine: Pour the dry Marsala wine directly into the hot skillet and use a wooden spoon to scrape up all the browned fond from the bottom of the pan. Increase the heat to medium-high and cook the wine for 3 minutes until the liquid reduces by half and the sharp alcohol aroma gives way to a round, slightly sweet fragrance. Pour in the chicken broth and stir to combine.
Reduce and Finish the Sauce: Continue cooking the sauce over medium-high heat for 4 minutes, stirring occasionally, until it thickens enough to coat the back of a spoon and a line drawn through it holds for 2 seconds. Remove the skillet from the heat and add the cold butter cubes, swirling the pan continuously until each cube melts fully into the sauce and produces a glossy, cohesive texture. Cold butter emulsifies the sauce without breaking it.
Return Chicken and Serve: Nestle the seared cutlets back into the skillet, spooning the Marsala mushroom sauce over each piece. Return the skillet to medium heat for 2 minutes until the cutlets are warmed through and the sauce clings evenly to the surface. Sprinkle the chopped flat-leaf parsley over the top and serve immediately from the skillet.
