Collect chicken drippings (if using): after roasting a chicken, pour the drippings into a measuring cup and let them settle. Skim off most of the fat. If you do not have chicken drippings, use chicken broth -- store bought or homemade. You need 2 cups of chicken drippings or broth.
Make a roux: heat a medium saucepan over medium heat. Add 4 tablespoons butter. When melted, slowly add in ¼ cup all-purpose flour and whisk constantly. Allow the roux to cook for 2-3 minutes, until it's lightly golden brown.
Add 2 cups chicken drippings or chicken broth. Slowly whisk in the chicken drippings or broth, a little at a time to prevent lumps. Whisk until smooth.
Simmer and thicken: bring the chicken gravy to a gentle simmer. As it simmers, it will thicken. If it gets too thick, add in a splash more of chicken broth, milk, or heavy cream.
Season: add in ½ teaspoon coarse black pepper and salt to taste. Optionally, add in ½ teaspoon each of garlic powder and onion powder. Taste, then adjust seasonings as needed.
Serve: pour gravy into a gravy boat or bowl and serve warm with roasted chicken, mashed potatoes, or even biscuits.