In order to thicken sauces with butter, a necessary condition is that the butter must be cold. A good way is to cut it into smaller cubes and place it in ice water before using it.

Adjust the intensity of the hotplate to the lowest degree. Add the cold butter in small portions, stirring constantly, waiting for it to be fully incorporated before adding the next portion. Add as much butter as needed so that the liquids boil to the desired degree according to your preferences.