In a stand mixer fitted with a paddle attachment, cream the softened butter and sugar on medium speed for about 2 minutes, until pale and creamy.
Scrape down the sides and bottom of the bowl with a rubber spatula, and mix for another 20 seconds.
Add the egg, vanilla paste, salt and lemon zest. Beat on low speed until the yolk is evenly incorporated and the mixture is clumpy, for about 30 seconds.
Add the sifted flour and mix on low speed for about a minute, until the flour is incorporated. The mixture will look like wet, clumpy sand.
Using your hand, lightly knead the dough into a ball inside the bowl. Do not overwork the pastry.
Turn the dough into a ball and roll it between two sheets of baking paper, about 3 mm (⅛ inch) thick. Place on a flat tray and leave to rest in the fridge for at least an hour.
Take the dough out of the fridge, remove the baking paper and cut out 10 rounds of pastry using a small knife. A round or fluted cookie cutter works well here.
Gently slide the pastries into the small tart tins, lightly pressing the edge between the bottom and the sides to remove any air bubbles. Mine are 8 cm (3 inch) tins. Ease the pastry in rather than pulling it to fit. Stretching it is what makes shells shrink in the oven.
Prick the bottom of the pastries with a fork to stop them rising, then put them back in the fridge to rest for another hour.