To make the sauce, first make sure the sucs have not burned. Add a touch more oil if needed, followed by the shallots. Let the shallots cook until soft and translucent.
Next, deglaze with the Marsala. Scrape the bottom of the pan to incorporate the sucs and let the wine reduce until almost all of the liquid has evaporated.
Next, add the dark chicken stock. Bring it to a simmer and let it reduce by about half. Add any juices from the resting chicken. Let the liquid reduce until you reach a sauce-like consistency.
To finish the sauce, swirl in the cold butter and taste for seasoning. Depending on the type of stock you have used, you may need to season with a bit of salt and pepper.