Prep the chicken: Pat the chicken breasts dry and season both sides with garlic powder, salt, and pepper. Dredge lightly in flour, shaking off any excess.
Sear the chicken: Heat 1 tbsp butter in a large skillet over medium heat. Add chicken and cook 3–4 minutes per side, until golden brown and cooked through. Remove from pan and set aside.
Cook the mushrooms & shallot: In the same pan, add the remaining butter. Toss in the sliced mushrooms and cook until browned, about 5 minutes. Add the shallot, garlic and thyme, cooking for another 1–2 minutes until fragrant.
Make the sauce: Pour in the Marsala wine to deglaze the pan, scraping up any browned bits from the bottom. Let it simmer for 2–3 minutes to reduce slightly. Add the chicken broth and stir, then finish with the heavy cream. Let the sauce thicken for 2–3 minutes. Taste and adjust seasoning with salt and pepper.
Combine & serve: Return the chicken to the pan, spooning the sauce and mushrooms over the top. Serve hot over a bed of creamy mashed potatoes, letting the sauce soak in.