Place the milk in a pan with the split vanilla pod and bring slowly to the boil. Remove from the heat, then cover and infuse for 20 minutes.

Whisk the egg, yolks and caster sugar together and add the milk. Return to a clean pan and cook gently without boiling, stirring, until the custard thickens. Strain into a bowl and cool for 1 hour, then chill for 3 hours. To prevent a skin forming, sprinkle the surface with caster sugar.

Lightly whip the cream until just stiff, then whisk in the chilled custard. Pour 1 tablespoon of Cassis into the bottom of four tall glasses. Place a sponge cake in each, then build up the layers starting with custard, then cherries and Cassis, finishing with the custard. Decorate with cherries, to serve.