To start the sauce, make sure to have the shallots minced and measure out the cold butter and red wine.
Over medium to medium low heat, add about one tablespoon of the butter, followed by the shallots. Cook the shallots until soft and golden. Once ready, deglaze with the red wine, making sure to scrape up any sucs from the bottom of the pan. Let the liquid reduce until you reach a syrupy consistency.
Once a syrupy consistency has been reached, add the dark chicken stock and any juices from the resting steaks and reduce again until slightly thickened. Test the sauce by spooning a bit onto a plate. The sauce should not be too runny or too thick. It should just move nicely on the plate.
To finish the sauce, turn off the heat and swirl in the cold butter, a bit at a time. The butter not only helps to thicken the sauce, it also adds richness and shine.
Finally, season the sauce with salt and pepper to taste.