Now wash and prepare your blackberries. If you want jam with chunky pieces of fruit in it (my personal preference) then lightly mash them with a potato masher in your pan. If you prefer smooth jam, place your blackberries in a food processor and blend them up before placing them in the pan. At this point you can seive them with a fine mesh seive if you prefer seedless jam
Add a tablespoon of vanilla essence and all of the jam sugar to the blackberries, and then over a medium heat, slowly heat up the mixture and stir until all of the sugar has dissolved. Do not let it boil at this point.
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.
Once set, give the pot a good stir, remove the jars from the oven, and ladle in the jam into the hot jars, working quickly. You may burn your fingers at this point - be warned!
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.