Cut the frozen butter and arrange it evenly on baking paper, wrap it to form a rectangle of about 10 cm x 15 cm.
Spread the butter to a thickness of about 3 mm using a rolling pin. Press firmly and distribute evenly.
Refrigerate the butter for 30 minutes.
Dice the cooled butter and mix it with all-purpose flour, salt, and sugar, using your fingers until you get a breadcrumb-like texture.
Add water and use a spatula to mix until you have a dough. Knead with your hands until no flour is visible.
Wrap the dough in plastic wrap and refrigerate for 30 minutes.
Take the dough out, fold it several times and roll it out into a rectangle the same size as the butter.
Place the butter on half of the dough, fold it in half and refrigerate for another 30 minutes.
Repeat the stretching and folding process two more times, refrigerating the dough for 30 minutes between each cycle.
Prepare the egg mixture by beating the eggs and sugar. Cover with cling film and refrigerate for about 1 hour until the sugar has melted.
Take the dough out, cut it into slices about 2 cm thick, obtaining about 12 portions.
Place the dough in tartlet pans, pressing the edge with your thumb and rotating the pan for even coverage.
Freeze for 10 minutes.
Line a baking sheet with foil (watch out for sharp edges).
Preheat the oven to 210°C.
Pour the egg mixture into the molds, filling them to about 80%.
Bake in the preheated oven on the medium position for 23-25 minutes.
Remove the tarts from the pan after removing them from the oven and leave to cool on a wire rack.
Enjoy the delicious meringue layers. Despite the complicated steps, the final product is worth it.
Advice:

Be careful when handling aluminum foil, as it can be sharp.
Make sure the dough is properly refrigerated between folding steps to get the best flaky layers.
Use caution and follow appropriate safety precautions when handling