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To make the custard, put the milk, cream and caster sugar in a saucepan and bring to the boil. Meanwhile, set a glass bowl over a pan of gently simmering water. Snap the white chocolate into the bowl and leave to melt. Put the egg yolks in a separate bowl.
Once the cream mixture is up to the boil, pour into the bowl with the yolks, whisking continually. Return the mixture to the pan and cook over a gentle heat, whisking until the mixture reaches 85C on a digital thermometer, or the custard has thickened enough to coat the back of a spoon and you can draw a line through it with your fingertip. Pour the custard through a sieve into the melted chocolate and quickly whisk to combine. Pour the white custard into a bowl, cover with cling film and chill in the fridge overnight.
The next day, crumble the cake into the bottom of a trifle bowl or individual glasses and pour over the rum and half the passion fruit pulp. Leave to soak for 10 mins, then spoon over the white chocolate custard. Put in the fridge while you whisk the cream.
Whisk the cream, vanilla, sugar and lime zest to soft peaks, then spoon into a piping bag with a star nozzle. Pipe on top of the custard, then top with the remaining passion fruit, coconut and a grating of white chocolate. Chill until needed. Can be made a day ahead and decorated just before serving.