Make this deliciously indulgent blackberry and vanilla jam
Sterilise your jars - see this handy guide on how to sterilise glass jars if you're unsure how to do it correctly.
Add the knob of butter, stir well until it melts in, and then bring to the boil.
Once the pot has started boiling furiously set a timer for four minutes, and let boil.
Once your four minutes is up, remove from the heat and test to see if your jam has set. To do this, I always place a teaspoon of jam on a plate, and then place it in the fridge for a minute or two. If the jam is still runny after being in the fridge then return the pot to the heat and boil for another two minutes, and repeat the test. Keep doing this - boiling for two minutes and testing - until your jam sets on the plate.
Wipe any spills off of the rim of the jar and sides with a clean warm cloth. Remove the lids from the water and screw on tightly.
Your jam should then be good for 6 months – keep in a cool dark place for best results.