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To make the posset, put the cream, sugar and both zests in a pan and bring to a simmer. Turn up the heat and bubble for 3 mins exactly, then remove from the heat and stir in the citrus juice. Sieve the mixture into a large trifle bowl and chill for at least 4 hrs, or overnight until set.
Scatter the madeira cake pieces over the posset layer.
To make the fruit layer, stir the marmalade and vodka together, pour over the chopped fruit and mix. Tip the mixture over the cake pieces and return to the fridge to chill.
Meanwhile, make the custard. Put the chocolate and custard in a pan stirring over a low heat and gently warm until the mixture is piping hot and the chocolate has melted. Leave to cool, stirring occasionally to stop a skin forming. Once cooled, pour over the fruit layer.
To make the mousse, put the cream and egg whites in separate bowls, adding a pinch of salt to the egg whites. Using a very clean electric whisk, beat the egg whites to stiff peaks, add the sugar and briefly whisk again. Whisk the cream until it just holds its shape, then fold in the yogurt.
Chop the chocolate, put it in a bowl set over a pan of simmering water and stir until melted, or melt in the microwave in short bursts. Leave to cool for 3-5 mins, then stir in the egg yolks.
Add a large spoonful of the egg whites to the chocolate mixture and stir to loosen. Add the remaining egg whites and the whipped cream. Fold everything together with a large metal spoon, keeping as much air in as possible. When there are no more visible streaks, dollop the mousse on top of the trifle. Decorate with your chosen toppings. Will keep in the fridge for up to three days without toppings.