1.5kg fresh plums, firm and slightly under-ripe
1.5kg granulated sugar
1 tbsp lemon juice
100ml cold water
1. Wash plums and remove stalks. Remove plum stones and place plums in a large pan along with the lemon juice and water.
Bring to the boil, stirring frequently. The particular variety of plums I was using stained my fingers a pretty gnarly shade of orange, so be aware of that. At this point you may or may not wish to sample a plum, just to make sure they are tippy-top.
2. Reduce heat and simmer for around 40 mins, until plums are soft.
3. Keep on a low heat and add the sugar. Stir continuously until sugar is fully dissolved.
4. Stir in the butter to reduce frothing. Turn up the heat and stir continuously until the mixture reaches a rapid boil.
5. Continue heating until jam reaches setting point. If you have a sugar thermometer, this is 106 degrees. If you don't have a thermometer, test it using the plate method described here.
6. When the jam is ready, remove it from the stove and pour it straight into sterilised jars. Make sure you have enough jars ready so that you don't have to improvise like I did...