Thaw the blackcurrants, then leave them to soak in the sink filled with cold water. Remove any stalks and excess leaves and place in a saucepan with water. Bring the mixture to the boil and allow to simmer while left uncovered for approximately 40 minutes, until the fruit is softened and the water has reduced. Next, add in the sugar and stir vigorously until the sugar has dissolved and bring it to a gentle boil. Let it bubble for 10 minutes then test for a set. Put a spoonful onto a chilled plate then place in the fridge for around 5 minutes.
In the meantime, turn the jam right down to a low heat. The sample from the fridge should have set into a jam like consistency if you touch it with your finger. If it has not set, return the mixture to the heat and let it boil, then test again. When properly set, add the butter and stir well. Cool the mixture for 10 minutes to prevent the fruit from sinking to the bottom of the jar. Ladle the jam into sterilised jars, but don’t fill to the top. Let the jam cool before you cover it.
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