Prepare Chicken:

• Set a wire rack inside a rimmed baking sheet.
• Cut each chicken breast in half horizontally to create 6 thin cutlets.
• Place one cutlet between two sheets of plastic wrap and pound it to an even 1/4-inch thickness (about 6-8 inches long). Repeat with remaining cutlets.
• Transfer the pounded cutlets to the wire rack and season generously on both sides with kosher salt and freshly ground black pepper.

Prepare Seasoned Flour: In a shallow bowl or dish, whisk together the all-purpose flour, 1 1/2 teaspoons of kosher salt, and a generous amount of black pepper.

Dredge Chicken: Dredge each chicken cutlet in the seasoned flour, coating both sides and shaking off any excess flour.

Sear Chicken (in Batches):

• Heat 3 tablespoons of the olive oil in a large skillet (cast iron recommended) over medium-high heat until shimmering.
• Carefully place 3 chicken cutlets in the hot skillet. Sear until golden brown, about 2 minutes per side. Transfer the seared cutlets to a plate.
• Add the remaining 1 tablespoon of olive oil to the skillet. Sear the remaining 3 chicken cutlets until golden brown, about 2 minutes per side. Transfer to the plate with the other chicken. (The chicken does not need to be fully cooked at this point; it will finish cooking in the sauce.)

Cook Mushrooms & Shallots:

• In the same pan (do not clean it), add the sliced mushrooms and 1/2 teaspoon salt. Cook over medium-high heat, stirring occasionally, until the mushrooms soften and release their liquid, about 3-4 minutes.
• Add the finely chopped shallot and continue to cook, stirring, until the shallot softens, about 3-4 minutes more.

Make the Marsala Sauce:

• Carefully remove the pan from the heat. Pour in the Marsala wine and the 1/2 cup of water.
• Return the pan to high heat. Bring the mixture to a boil, scraping up any browned bits (fond) from the bottom of the pan with a wooden spoon or spatula.
• Reduce the heat to medium and simmer until the liquid reduces by about one-third, about 6-7 minutes.
• Reduce the heat to low. Whisk in the butter pieces, one at a time, until the butter is melted and the sauce thickens slightly, about 1-2 minutes.

Finish the Dish:

• Return the seared chicken cutlets (and any accumulated juices from the plate) to the skillet with the sauce.
• Simmer gently for about 2 minutes, spooning the sauce over the chicken, until the chicken is cooked through (internal temperature reaches 165°F/74°C) and the sauce has thickened slightly.

Serve: Transfer the chicken cutlets to a serving platter. Stir the 1/3 cup of chopped fresh parsley into the sauce in the skillet. Pour the sauce over the chicken. Garnish with additional chopped parsley, if desired. Serve immediately.