Start by making the jelly. Put the blackcurrants (there's no need to top and tail them first) in a small saucepan with the apple juice. Warm to a gentle simmer until the currants break up and release all of their dark purple juices (you can do a bit of squishing with a spoon to help with this if you like).
Pour the mixture into a silicone ice cube tray and leave to set for around 8 hours (or overnight) in the fridge. We find that a short stint in the freezer after the jelly has set (30 minutes should be plenty) is a good way of firming up the cubes so that they are easier to pop out of the tray and add neatly to the sponge filling without breaking.