Strain the drippings through a fine-mesh sieve. Let them sit until the fat rises to the top, then scoop off any excess fat than you can. If needed, add chicken,turkey broth or water to top up the drippings to 4 cups.
Melt the butter in a large saucepan over medium heat. Sprinkle in the flour and cook, whisking continuously, until it browns lightly and begins to smell toasted, around 3-4 minutes. You need to brown the flour for the best taste!
Slowly pour in the pan drippings, whisking continuously.
Bring to a boil then reduce the heat and simmer, whisking constantly, until thickened, about 5-10 minutes.
If needed, thicken with a cornstarch slurry to desired thickness.
Season with salt and pepper, to taste.
Serve warm.
Notes
Depending on your drippings, you might have to use the cornstarch slurry to thicken. Do not add more flour, as it needs to be toasted to have that rich flavor in the gravy.