Put the blackberries and sugar into a small pan with 100ml/31⁄2fl oz water. Bring to the boil, then simmer for 5 mins until the fruit is soft. Stir in the vanilla, remove and cool a little.
Tip the contents of the pan into a blender or food processor, and whizz to a purée, then strain through a sieve, rubbing it through with the back of a ladle or spoon. Serve warm or chilled. Keeps in an airtight container in the fridge for up to 3 days, or freeze for up to 3 months.
Making it red!
For a Raspberry & redcurrant coulis, simmer 140g/5oz redcurrants with 2 tbsp caster sugar and 4 tbsp water until the fruit bursts. Cool, then whizz in a food processor with 250g/9oz raspberries. Strain.