Let's start with the dough. To begin with, put the flour and sugar in a large mixing bowl and then slowly add 150 ml of ice-cold water. If you feel that is too much, just add enough water in order to make a soft dough that picks up the crumbs from the outside of the bowl without being overly sticky.
After you knead the dough, wrap it in cling film and let it chill in the fridge. Next, roll the dough onto a floured surface in order to create a rectangle. In order to get an idea, the rectangle should be about 30x40cm in size.
Now spread half of the soft butter over the dough. Make sure you spread it in a way that creates an even layer. Take the shorter edge of the rectangle that faces you and fold the top third of the dough towards you to the center. Then, grab the bottom third and fold it over. Now give the dough a quarter turn and roll it out to the same size as before.
After that spread the rest of the butter over it and repeat the process. Wrap the dough in cling film again and let it chill in the fridge for a few hours or, best, overnight.
Now, it's time to make the filling cream. Take a large mixing bowl that is also heatproof and put the 20 quail egg yolks and cornflour in it. Next, grab the sugar and whisk it in until the mixture is pale yellow and thick.
Now take the vanilla pods and split them in half along the length. Make sure to scrape out the seeds and put them in a medium-sized saucepan along with the empty pods, cinnamon stick, and lime zest. Now pour the milk and cream over these ingredients and stir.
Heat the mixture up very slowly until it's simmering, just like you would for quail egg creme brulee. At this point, pour the simmering mixture over the quail egg yolks and whisk well. Next, pour the custard back into the pan and warm-up for about 3 minutes. Then pour it into a cup or jug and cover it with a lid or cling film.
Time to bake the tartlets. Turn the oven on and set the temperature to 200°C or 390°F or just let it heat up for 15-20 minutes if you're using a gas oven. Take a 12-hole muffin tin and grease it with butter. Now, get the dough out of the fridge and roll it flat to about 40x20cm.
Grab the longest side and roll it up tightly in order to create a long roll of pastry. Trim the ends off and cut the roll into 12 pieces. Next, turn those pieces cut-side up on your work surface so that the spiral is facing upwards.
Then, flatten them out carefully until they are large enough to line the holes in the muffin tin. Try not to skew the spiral shape too much. (You can also skip this step if you don't really care for decor.) Press the dough into the muffin tins and pour in the custard mixture. Make sure you fill the tarts almost all the way to the top!
Reduce the heat to medium and place the Portuguese tartlets in the oven. Bake them for about 30 minutes or at least until the pastry is cooked through and the custard puffs out.
When the baking time is up, take the Portuguese quail egg custard tarts out of the oven and let them cool down. Then, put them in the fridge and allow them to chill for 2 hours. Serve them cold.