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.
Using your hand, lightly knead the dough into a ball inside the bowl. 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, roll it between two sheets of baking paper, about 3 mm (⅛") thick. Place on a flat tray and leave to rest in the fridge for an hour.
Let the ganache cool down just a little (for a few minutes), and pour it over the tartlet shells. Use a small spatula to spread it evenly if necessary.