Clean the fruit and remove the stones. You can slightly crush the stones and put them to a muslin bag to add to the jam as you are making it. The plum kernels and stone help with the jam flavour and also contain pectin. Just make sure you remove the bag from the jam before potting
First of all put all the fruit in a preserving pan (or pan large enough to handle about 2-3 ltr of volume), add 350 ml of water, cover with a lid and simmer over low to medium heat until the fruit becomes soft. This can take about 20 minutes
Remove the lid and increase the heat and boil for about 10-15 minutes or until the volume has reduced by about 2/3. At this stage the plum jam might start sticking, so gently stir the mixture to prevent this.
Now add the sugar and plum kernels (if using) and heat gently to dissolve the sugar. Add the butter and bring to boil. Keep the jam as a 'rolling boil' for 10-15 minutes, stirring occasionally. If you are using jam thermometer (you don't need to have one, but it's sometimes helpful) the temperature should reach about 105 C.
Take the pan off the heat.
To make sure that your plum jam is done do the 'cold saucer test'. Before you start your jam making put at least 2 small plates to fridge. When you are ready to test the jam, take one plate out pour a little of the jam on the plate and put back to fridge. If in 5 minutes, the jam is wrinkling as you are pushing it with your finger, then you know the rest of the jam will set too. If the jam didn't set, just put the pan back on to the stove and boil for another 5 minutes. Test again.
If you, like me, sometimes forget to put enough plates to the fridge, just quickly stick one to freezer before you do the test.Leave the jam to cool a little and then pot.