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In advance, put the prunes into a Kilner jar or a bowl with a lid. In a saucepan, combine the sugar, vanilla pod and 230ml water. Boil until the sugar has dissolved, then pour over the prunes. Leave to cool, then add the Armagnac. Put the prunes in the fridge to macerate, ideally for at least 24 hours.
When you’re ready to make your trifle, put most of the biscuits in a deep dish, reserving a handful to crush on top, and splash the amaretto on top. Give it some time to sink in, shaking occasionally so that everything is boozy.
Put a scant layer of the Armagnac-soaked prunes on top of the biscuits. Whip the cream with the Marsala and vanilla extract until it’s perfectly floppy, then spoon on top of the prunes. Crush over the remaining biscuits.
Cover the trifle with cling film and leave in the fridge until you are ready to serve. Dig in with a large spoon.