Stir the yeast into the lukewarm milk and water with a pinch of sugar and leave for 10 minutes until frothy.
Mix the flour, salt and sugar in a large bowl, pour in the yeast liquid and knead for 5-6 minutes into a smooth, elastic dough (don't overwork it, or it will be hard to roll).
Cover and leave to rise at room temperature for 1 hour, then flatten it, wrap in cling film and chill for at least 1 hour (or overnight).
Put the cold butter between two sheets of baking paper, beat it with a rolling pin and flatten it into a square of about 18×18 cm, then return it to the fridge — the butter and dough should be similarly cold and pliable.
Roll the dough out on a floured surface into a square of about 26×26 cm, put the butter slab in the middle turned 45°, fold the corners of the dough over it like an envelope and pinch them together.
Roll it into a long rectangle of about 20×60 cm, then fold it in three like a letter (the first 'tour'). Wrap in cling film and chill for 30-45 minutes.
Repeat the rolling and folding in three twice more, turning the dough 90° each time and chilling for 30-45 minutes after each turn. If the butter threatens to break through the dough, chill it again straight away.
After the last chill, roll the dough into a strip about 4 mm thick and 25 cm wide and cut it into long isosceles triangles (base about 9-10 cm).
Make a 1 cm cut in the middle of the base of each triangle, then roll up loosely from the base towards the tip and place tip-side down on a tray lined with baking paper, curved into a crescent or left straight.
Cover and leave to rise in a draught-free place no warmer than 26 °C for 1.5-2 hours, until doubled and the layers are clearly visible (somewhere warmer, the butter will leak out).
Preheat the oven to 200 °C. Mix the egg with the milk and brush the croissants carefully, avoiding the cut edges of the layers.
Bake for 15-18 minutes until deep golden and crisp. Cool on a rack for at least 15 minutes — they're best lukewarm.
A Receptnaptár original recipe. Calories and protein are estimated.