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Sterilise the jars and any other equipment before you start (see tip, below). Put a couple of saucers in the freezer, as you’ll need these for testing if the jam is ready later (or use a sugar thermometer). Put the berries and juice (or water) in a preserving or very large deep pan, bring to a simmer, then cook for about 15 mins until the fruit is very tender.
Add the sugar to the pan and heat gently until all the sugar has dissolved – do not boil at this point. Turn up the heat and cook the jam at a rolling boil, skimming away any scum that comes to the top as it cooks. After about 10 mins, spoon a little jam onto a cold saucer. Leave for a few secs, then push the jam with your fingertip. If it wrinkles, the jam is ready. If not, cook for a few mins more and test again on another cold saucer. (If you have a sugar thermometer, it will read 105 degrees when ready.)
Give the jam a final skim, stir in the butter, then the vanilla seeds and the pod too. Let the jam cool for a few mins – this will thicken it and prevent any lumps from sinking to the bottom of the jars – then ladle into hot jars, seal and leave to cool.