The night before, layer the blackberries and sugar in a big bowl, then cover it and set it aside at room temperature. This allows the sugar to start dissolving in the fruit juices and gives you a head start so you don’t overcook the fruit or burn everything. When you’re ready to start making your jam, just give everything a stir before you start.
Take a saucer or small plate and place it in your freezer to cool. Grab your preserving pan or your largest wide-based pan and tip in the blackberries and sugar, making sure to scrape out all the sugary juices. Stir in the lemon juice and then collect all the pips, which contain more pectin, and secure them in a muslin square or a tea-leaf ball and add them to the pan.
Over a low heat, gently warm the blackberries until all the jam is completely dissolved, then bring the pan to the boil and simmer for 5 minutes. Remove the pan from the heat and spoon a little hot syrupy jam onto your chilled saucer. Give it a minute or two to cool, then push it with your finger. If it wrinkles a bit it has reached its setting point and it’s ready; if not, put the saucer back in the freezer and the pan back onto the heat and boil the jam for 2-3 minutes then remove from the heat and try again.
Skim off any excess scum that will have formed and stir in the knob of butter to dissolve any that remains. Leave the jam in the pan for 15 minutes before ladling into sterilised jars; this pause allows the fruit to settle in the jam and prevents it from sinking to the bottom. Carefully wipe the necks of the jars and fit the lids or seal with a circle of waxed paper and a fabric square tied on with string. The jam will keep in a cool dark place for at least six months, and should be refrigerated once opened.