Step 1Pour out all the fat in the roasting pan except for about 1 tablespoon. Place the roasting pan on the stove (over 2 burners, if possible) over low heat. Add the shallots to the pan and sweat about 3 to 5 minutes.
Step 2Turn off the heat, then pour in the Cognac or brandy. Increase the heat to high. Stir with a wooden spoon, scraping up all the browned bits from the bottom of the pan. Reduce the mixture until it's syrupy, about 3 minutes. Add the demi-glace, stir to incorporate.
Step 3Strain the sauce into a small saucepan. Warm it on low heat until it just starts bubbling, then remove from heat and whisk in the butter, a few small bits at a time. Add salt and pepper to taste, then pour into a sauceboat and serve immediately.