Cut the plums in half and carefully take the stones out.
Place them in a large pan and add the water and bring to the boil over a medium heat.
Once boiling, reduce the heat and allow the pan to simmer for 40 minutes so the fruit can soften. Stir occasionally.
This is when you need to add the sugar and lemon juice, when you do keep stirring until the sugar is fully dissolved.
Then split your vanilla pod and scrape out the seeds and add them to the fruit cut the empty pod in half and add this to the fruit also. Stir the vanilla in well.
Add the butter and mix it in, this will help keep the foaming and frothing down during the high temperatures.
Continue stirring until the jam reaches setting point 220F / 104C if you don’t have a thermometer you can gage this because the jam will start to stick to the sides of the pan.
Once the jam reaches temperature, or the jam is starting to stick to the sides of the pan and your wooden spoon it is time to test it to see if it is ready.
Take the pan off the heat and place a small amount of the jam on a small cold plate and allow it to go cold. Once it is cold run your finger through it, if it is set the jam will wrinkle if you feel it is too runny place the pan back on the heat and boil again for a few minutes and try the test again.
Once ready remove from the heat and carefully pour into clean, warm sterilised jam jars. If you see any air bubbles remove them. Seal and label the jars while they are still warm. Allow them to cool completely then store.