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The night before you make your jam, layer the blackberries and sugar together in a very large bowl, then cover and set aside at room temperature. This helps the sugar to start dissolving so you don’t run the risk of over-cooking the fruit when you actually begin to make the jam. The next morning, give everything a quick stir, then set aside again until you are ready to start cooking.
Before you start, put a small saucer in the freezer. Take a preserving pan or a large, wide-based pan (the wider and more open the pan, the faster the jam will be ready, so a preserving pan is really ideal) and tip the berries in, scraping out all the juices and any undissolved sugar. Stir in the lemon juice, then collect all the pips and secure them inside a tea-leaf strainer or piece of muslin before adding them to the pan (cooking the pips along with the fruit extracts the pectin they contain, helping your jam to set).
Start the blackberries over a low heat until all the sugar is completely dissolved, then bring to the boil and simmer for 5 mins. Turn off the heat and spoon a little hot syrupy jam onto the chilled saucer. Once it's cool, push it with your finger. If it wrinkles a little, it’s ready and has reached setting point. If it's too runny to wrinkle, return the pan to the heat and boil in 2 or 3-minute stages, removing the pan from the heat each time you do the saucer check, until the jam wrinkles.
Skim off any excess scum, then stir in the knob of butter, if you want – this will help to dissolve any remaining scum. Leave the jam for 15 mins before ladling into sterilised jars – this allows the fruit to settle so it doesn’t sink to the bottom. The jam will keep in a cool, dark place for at least 6 months. Refrigerate jars once opened.