This is a jam, not a jelly. Put the fruits and the sugar into a preserving pan. Bring to a boil over a moderate heat, shaking the pan from time to time. When the sugar has melted, stir, keep simmering gently. As soon as the froth becomes abundant, add the butter. It will push the froth to the sides, skim thoroughly. Cook until the juice falling on a cold plate sticks and does not run anymore. Remove from the heat. The mixture should be warm but not hot when added to the jars to prevent the fruit from falling to the bottom. Cover the day after.
