The next day, mix well, add the knob of butter, and then place the pot over high heat. Bring to a boil, stirring occasionally.
If despite the butter, a small foam forms, remove it with a skimmer at the end of the jam cooking.
Cook for 20 minutes from the boiling point. Also, check the texture, which should be thick and translucent.
You can use a potato masher to refine the texture of the jam.