Instructions

1. Put a small plate in the fridge.
2. Put the blackberries and sugar in a heavy-based saucepan. 
3. Turn on the heat and gently simmer the jam until it reduces to a thick, sticky jam, stirring occasionally. It should take about 20-30 minutes (if you add more sugar, the cooking time will be much less).
4. Drop a little jam onto the cooled plate and put it in the fridge for 30 seconds, then run your finger through it. If it forms a crinkle and is gluey it’s done. If not, cook for a couple of minutes and retest until it’s ready.
5. Discard any scum with a spoon, then mix in in the vanilla paste and lemon juice.
6. Spoon the jam into a small sterilised jar (about 200 ml) and seal with a lid immediately.
7. Let it cool, then keep it a cool place (such as larder, pantry, cellar). After opening, keep it in the fridge.