Combine 1 tablespoon of cornstarch and 2 tablespoons chicken broth in a small bowl to make a slurry. Set aside.
Combine remaining cornstarch and Essence in a ziploc bag. Set aside.
Trim chicken breasts of all fat. Horizontally slice chicken breasts into two to three thin slices. Place in ziploc bag. Shake to thoroughly coat.
Heat the oil in a large skillet over medium-high heat until hot, but not smoking. Cook the chicken breasts until golden brown on both sides, about 3 to 4 minutes per side. Transfer to a plate lined with paper towels and set aside.
Add the mushrooms to the skillet; cook over high heat, stirring frequently, until the mushrooms are golden brown around the edges and have given off their liquid. Add the Madeira and bring to a boil, scraping to remove any browned bits from the bottom of the pan. When the Madeira has reduced by half, add the remaining chicken stock and cook for 3 minutes, or until the sauce has thickened slightly. Add the cornstarch/chicken broth slurry; cook stirring constantly until thickened.
Lower the heat to medium. Add the sour cream and stir until well combined. Return the chicken breasts to the pan. Continue to cook until the chicken breasts are heated through, about 2 to 3 minutes. Season to taste with salt and pepper. Garnish with chopped chives or green onions and serve immediately. Yield: 4 servings.