These are traditional Portuguese custard tarts, flavoured with lemon and cinnamon.
For the pastry, mix the flour and salt together in a bowl. Rub in the chilled butter, until the mixture resembles breadcrumbs. Gradually add enough chilled water (about 4–6 tablespoons) to form a dough.
Roll out the dough to a rectangle on a lightly floured work surface. Grate half of the frozen butter over the bottom two thirds of the dough. Fold down the top third and fold up the bottom third as if folding a letter.
Turn the folded dough through 90 degrees and roll it out into a rectangle again. Repeat the process, grating in the remaining frozen butter and fold as before. Wrap the dough in cling film and leave to rest in the fridge for 30 minutes.
Roll and fold the pastry twice more, each time wrapping the dough in cling film and leaving it to rest in the fridge for 30 minutes.
Roll out the pastry on a lightly floured work surface to a rectangle measuring 20 x 30cm. Roll up the pastry tightly, from the short side, into a log and cut the log into 12 equal discs.
Place 1 disc into the cup of the muffin tin, swirl-side up. Using wet fingers, carefully press the pastry up the sides, working from the centre outwards, until the pastry just pokes over the top. Repeat with the remaining pastry discs. Chill for 20 minutes.
For the custard, pour the milk into a pan and whisk in the flour. Add the strips of lemon zest and the cinnamon stick. Bring to a simmer over a low heat, whisking continuously. Cook for 2–3 minutes until thick. Remove from the heat.
Dissolve the sugar in 185ml of water in a small pan over a low heat. Then, increase the heat and boil until a drop of syrup placed in a glass of water forms a thread (or it reaches 106–112°C on the sugar thermometer). Gradually whisk the boiling syrup into the milk mixture.
Put the egg yolks in a large bowl and strain the milk mixture over the top, whisking to combine. Cover the surface with cling film and leave to cool. Once cooled, remove the lemon zest and cinnamon stick.
Heat the oven to 240°C/475°F/Gas 8 (don’t use a fan oven). Pour the custard into the pastry cases, leaving a 1cm gap at the top, then bake for 15–18 minutes, until the pastry is golden and crisp and the custard is bubbling with tiny brown spots.
Cool the tarts in the tin for 5 minutes, then gently transfer to a wire rack to cool completely.