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, and beat on low speed until the yolk is evenly incorporated and the mixture is clumpy, for about 30 seconds.
Add the 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. You can transfer the dough to your worktop to knead it if easier, but make sure not to overwork the pastry.
Turn the dough into a ball and then roll between two sheets of baking paper, about 3 mm (⅛") 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. You can also use a round or fluted cookie cutter.
Gently slide the pastries into the small tart tins, lightly pressing the edge between the bottom and the sides to remove any air bubbles.
Prick the bottom of the pastries with a fork to avoid them rising, then place them back in the fridge to rest for another hour.
Preheat your oven to 170° C (340° F) and place the tart tins in the freezer in the meantime.
Line each tartlet shell with a small piece of cling film or baking paper and cover with baking beads, beans or chickpeas.
Bake the shells for 10-12 minutes, remove the baking weights and cling film carefully, then bake for another 15 minutes or until golden brown.
Leave them to cool down completely before adding the filling.
Proceed to prepare the chocolate ganache while the pastry shells are cooling down.