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Measure 100ml passion fruit pulp into a jug (reserving the rest), add the lime juice, stir, then set aside. Combine the cream and sugar in a medium saucepan set over a low heat and stir for 2 mins to dissolve the sugar. Turn the heat up to medium and bring to the boil. Simmer for 2 mins, then remove from the heat and immediately stir in the passion fruit and lime juice mixture. Divide evenly between six 175-200ml cups or glasses – do this quickly, as the mixture will start to thicken. Leave to cool for 10 mins, then chill for at least 4 hrs or overnight. The possets can be made up to three days ahead and chilled.
To make the biscuits, heat the oven to 180C/160C fan/gas 4 and line a large baking tray with baking parchment. Beat the butter and icing sugar together for 1 min in a medium bowl using a wooden spoon. Stir in the custard powder, flour, vanilla (if using) and chopped pistachios to make a soft dough. Roll the dough into 12 walnut-sized balls and arrange on the baking tray, well-spaced out. Flatten the balls slightly with your palm or the base of a glass, then press down in the centre of each with the tines of a fork. Bake for 10-12 mins until very pale golden. Leave to cool on the tray for 10 mins then transfer to a wire rack to cool completely. The biscuits will keep in an airtight container for five days.
Spoon the remaining passion fruit pulp over the possets and serve with the biscuits and pineapple slices.