To make the pastry, sift together the flour, icing sugar and salt and place in a food processor. Add the butter and lemon or lime zest and pulse a few times, until the mixture has the consistency of fresh breadcrumbs. Lightly whisk together the egg yolk and water and add this to the mix: the dough will feel quite wet, but this is as it should be. Process once more, just until the pastry comes together, then tip on to a clean, lightly floured work surface. Press or pat gently to form a ball, then divide the pastry in two. Wrap each half loosely in cling film and press gently to form two flattish discs. The pastry is very soft so you need to keep it in the fridge for at least 1 hour (or up to 3 days).
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Brush the moulds of the mince pie or regular muffin tin with melted butter (see Kit Note) and set aside.
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When ready to bake, allow the pastry to rest at room temperature for 30 minutes (if it has been in the fridge for more than a few hours) and place on a lightly floured work surface, working with one disc of pastry at a time. Tap all over with a rolling pin to soften slightly before rolling out until 2mm thick. Using a 7cm cookie cutter, stamp out eighteen circles and place one in each greased mould. Re-roll the pastry, if necessary, until you get eighteen circles. Set aside in the fridge to rest. The remaining pastry disc can be frozen for future use.
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Press the pastry offcuts (from the used disc) together and roll into a rough rectangle, about 2mm thick. Cut the pastry into strips, about 6 x 0.5cm: these will form the lattice to decorate the tarts. Transfer to a baking tray and keep in the fridge until ready to use.