900 grams blackberries
500 grams sugar
1 ½ tablespoon fresh lemon juice
teaspoon of butter
Put the fruit into a large heavy-based saucepan, add 50 ml of water and the lemon juice and bring to a boil. Lower the heat and simmer for 20 minutes until the fruit softens. Put in the sugar and gently stir over a very low heat until the sugar has completely dissolved. Raise the heat, bring to a rolling boil, and then boil for about 20 minutes, skimming off the foam as you go, but don’t stir the jam.
Test for setting point by spooning a little bit of jam onto a chilled plate. After a minute, push your finger through the jam on the plate and if the surface wrinkles, it’s ready; if it doesn’t, keep cooking for 2-minute intervals, testing in between. (Or if you have a sugar thermometer it should reach 105˚C.)
Remove from the heat, skim off any excess foam and rub a knob of butter across the surface. Leave for about 15 minutes, then pour into sterilised jars and seal.