If using pan drippings, pour the contents through a strainer or fine-mesh sieve over a bowl, so that the drippings end up in the bowl. Discard the solids.
Add all the ingredients except the xanthan gum mixture and butter. Bring to a gentle boil.
Add the xanthan gum mixture to thicken. Whisk constantly until thickened, 2 to 4 minutes.
Remove from heat and swirl in cold unsalted butter for a smooth, velvety finish and richness.