Put the blackcurrants into a bowl and mash thoroughly. Add the caster sugar and mash again. Scrape the purple mush into a large sterilised kilner jar, drop the leaves in and pour over the brandy. Stir well.
Leave on a warm windowsill for a week, giving it an occasional stir, then fish out the blackcurrant leaves and put the jar in a cool dark location. Allow to steep or macerate for three months then strain out the fruit – boozy blackcurrants make a great tart or crumble when combined with cooking apples – and pour the resulting liquid into sterilised bottles.Allow to mature for at least another three months. Makes 900mls of cassis, approx.