Put the plums in a roasting tin. Sprinkle over the sugar and add 65ml water. Cover with foil and bake for 35 minutes, until the plums are just soft but still holding their shape. Set aside to cool then drain off the juice into a bowl.
Dip each sponge finger into the plum juice then lay them in the bottom of a 2-litre glass bowl. Spoon the cassis over the top. If the sponge isn’t wet enough, spoon the rest of the plum juice on top.
Lay the plums on top, cover with cling film and put in the fridge while you prepare the rest.
For the jelly, heat the pomegranate juice, orange juice, sugar and pomegranate molasses gently, stirring to help the sugar dissolve. Remove from the heat.
Put the gelatin leaves in a bowl and add enough cold water to cover. Let the leaves soak until they are soft – about 10 minutes. Squeeze out using your hands then add them to the juice, which should be warm but not hot (too cold and the gelatin won’t dissolve; too hot and its setting properties will be ruined).
Stir until the gelatin has melted. Transfer to a bowl and leave until the jelly has started to set – it should be soft but just holding together – then pour this over the plums. If you add the jelly before it’s starting to set, the plums and sponge fingers will start to float in the jelly – not the look you want. Cover the bowl with cling film and put in the fridge to set.
For the custard, put the egg yolks, sugar and cornflour in a large bowl and mix well. Heat the milk, cream and vanilla in a large saucepan until just below boiling, then remove from the heat and slowly pour them over the sugar and egg mixture, whisking continuously.
Clean the pan, put the egg and cream mixture into it and heat over a medium-low heat for 2-3 minutes, stirring all the time, until thickened. If any lumps appear, whisk the mixture or beat hard with a wooden spoon until it’s smooth. Pour into a large bowl and cover the surface with cling film. Leave to cool.
Pour the custard over the jelly, cover the bowl and return to the fridge. It’s best to leave the trifle for a day before you add the cream.
Whip the cream until it’s just holding its shape. Stir in the remaining sugar and cardamom then, beating again, add a little lemon juice. Spoon the cream on to the surface of the custard. Don’t refrigerate – the cream sets and goes claggy – but serve immediately, scattered with pomegranate seeds.